Gangaridai was the name of a kingdom in 300 BC in what is now the Bangladesh and West Bengal region of South Asia. It was described by the Greek traveller Megasthenes in his work Indica. The Greek and Latin historians suggested that Alexander the Great withdrew from India anticipating the valiant counter attack of the mighty Gangaridai and Prasii (Nanda) Empires which were located in the Bengal region. However, not much is known about the origin of these empires.
Alexander Cunningham the doyen of Indian archeology and ancient history in his book The Ancient Geography of India translated Megasthenes Indica and Ptolemy 's periplus and termed the name Ganda-ridae. He identified the place in Punjab near Lahore or to be more accurate the ancient capital Taki. He mentions the place in connection to Ganda-bar or the central plain of any doabs formed by the five rivers in Punjab which are also the the five mouths of Ptolemy. This term or place had nothing to do with Bengal. The other alternative offered by him and also by Dr. RC Majumdar in his history of India mentions the Five tribes Panchala people who did inhabit the other famous doab the Ganga Yamuna doab. Prassi or Prachya means East but actually also means people of the South Bank and Gangahridaya if taken to be such instead of actual term Gandaridae in Megasthenes Indica means Heart of Ganga which again means the Prayaga Allahbad to Magadha area. Prassi and Gandaridae means people of Doab and people to the south bank. If Prassi is Panchala then Gandaridae are the people of Kosala. If Gandaridae is Panchala then Prassi is Yaudheya and other republican tribes to south bank of Yamuna which did in 100 AD drive out the other invaders Kushanas (Dr. Majumdar And Dr Altekar History of India.) The main point which all Gangahridaya historians try to overlook is that Arrian and Magasthenese describe Alexander 's campaigns in Gedrosia (Baluchistan) where the Greeks fought and pursued the Prasii. Baluchistan has tradition of Pratarajas or Prachyarajas. Sandrocottus or Chandragupta is called the King of Prasii who lead the rebellion. Chandragupta united the republican tribes of the Vaahika pradesh (or aratta aka non king country) as Prasii and attacked the Greeks eventually taking Gedrosia from them. So Prasii are people from banks of Indus to Ganges which hardly means Gangahridaya were so far east as Bengal.
Ptolemy (c. 90 – c. 168), writes that the Gangaridai occupies the entire region about the five mouths of the Ganges and that the royal residence was in the city of Gange. The five mouths are: 1. The Kambyson 2. The Mega 3. the Kamberikhon 4. the Pseudostomon 5. The Antibole.
The Periplus refers Gangaridai to be located on the Bay of Bengal north to the port city of Dosarene in Kalinga. And its main city with the same name as the river Ganges was on the bank of the river. Strabo, Pliny, Arrian et al.compiled a map of India as known to the early Greeks, based on ‘Indica’ of Megasthenes (4th century BC) where the Gangaridae kingdom has been shown in the lower Ganges and its tributaries. However all the Greek, Latin and Egyptian accounts about Gangaridai suggest that the country was located in the deltaic region of Southern Bengal.
Periplus mentions the city of Pataliputra which is north of Tosali or Dosarne and which if you look at the map, lies next to Ganges and is at the heart of Ganga as it flows from Himalayas to the Sea.
"When he (Alexander) moved forward with his forces certain men came to inform him that Porus, the king of the country, who was the nephew of that Porus whom he had defeated, had left his kingdom and fled to the nation of Gandaridai... He had obtained from Phegeus a description of the country beyond the Indus: First came a desert which it would take twelve days to traverse; beyond this was the river called the Ganges which had a width of thirty two stadia, and a greater depth than any other Indian river; beyond this again were situated the dominions of the nation of the Prasioi and the Gandaridai, whose king, Xandrammes, had an army of 20,000 horse 200,000 infantry, 2,000 chariots and 4,000 elephants trained and equipped for war".... "Now this (Ganges) river, which is 30 stadia broad, flows from north to south, and empties its water into the ocean forming the eastern boundary of the Gandaridai, a nation which possesses the greatest number of elephants and the largest in size. "----Diodorus Siculus (c. 90 BC– c. 30 BC). Quoted from The Classical Accounts of India, Dr R. C. Majumder, p. 170-72/234.
"Among the southern countries the first under the Kaukasos is India, a kingdom remarkable for its vast extent and the largeness of its population, for it is inhabited by very many nations, among which the greatest of all is that of the Gandaridae, against whom Alexander did not undertake an expedition, being deterred by the multitude of their elephants. This region is separated from farther India by the greatest river in those parts (for it has a breadth of thirty stadia), but it adjoins the rest of India which Alexander had conquered, and which was well watered by rivers and highly renowned for its prosperous and happy condition. "---Diodorus Siculus (1st century AD). Quoted from Ancient India as Described in Classical Literature, John W. McCrindle, p. 201.
"Next came the Ganges, the largest river in all India, the farther bank of which was inhabited by two nations, the Gangaridae and the Prasii, whose king Agrammes kept in field for guarding the approaches to his country 20,000 cavalry and 200,000 infantry, besides 2,000 four-horsed chariots, and, what was the most formidable of all, a troop of elephants which he said ran up to the number of 3,000. "---Quintus Curtius Rufus (wrote between 60-70 AD). Quoted from The Classical Accounts of India, p. 103-128.
"The Battle with Porus depressed the spirits of the Macedonians, and made them very unwilling to advance farther into India... This river (the Ganges), they heard, had a breadth of two and thirty stadia, and a depth of 1000 fathoms, while its farther banks were covered all over with armed men, horses and elephants. For the kings of the Gandaritai and the Prasiai were reported to be waiting for him (Alexander) with an army of 80,000 horse, 200,000 foot, 8,000 war-chariots, and 6,000 fighting elephants. "---Plutarch (42-120 AD). Quoted from The Classical Accounts of India, p. 198.
"Now this river, which at its source is 30 stadia broad, flows from north to south, and empties its waters into the ocean forming the eastern boundary of the Gangaridai, a nation which possesses a vast force of the largest-sized elephants. Owing to this, their country has never been conquered by any foreign king: for all other nations dread the overwhelming number and strength of these animals. [Thus Alexander the Macedonian, after conquering all Asia, did not make war upon the Gangaridai, as he did on all others; for when he had arrived with all his troops at the river Ganges, he abandoned as hopeless an invasion of the Gangaridai and India when he learned that they possessed four thousand elephants well trained and equipped for war. ]"---Megasthenes (c. 350 BC-290 BC). Quoted from the Epitome of Megasthenes, Indika. (Diod. II. 35-42. ), Ancient India as Described by Megasthenes and Arrian. Translated and edited by J. W. McCrindle.
"In the final part of its Ganges course, which is through the country of the Gangarides.... But Prasii surpass in power and glory every other people, not only in this quarter, but one may say in all India, their capital Palibothra (Pataliputra), a very large and wealthy city, after which some call the people itself the Palibothri, (He talks about Prasii during the reign of Chandragupta Maurya)... In the parts which lie southward from the Ganges the inhabitants, already swarthy, are deeply coloured by the sun, though not scorched black like the Ethiopians. "---Pliny the Elder (23-79 AD). Quoted from The Classical Accounts of India, Dr. R. C. Majumdar, p. 341-343.
"The least breadth of the Ganges is eight miles, and its greatest twenty. Its depth where it is shallowest is fully a hundred feet. The people who live in the furthest-off part are the Gangarides, whose king possesses 1,000 horse, 700 elephants, and 60,000 foot in apparatus of war. "---Megasthenes (INDICA) Quoted from FRAGM. LVI. B. Solin. 52. 6- 17. Catalogue of Indian Races, Ancient India as Described by Megasthenes and Arrian. Translated and edited by J. W. McCrindle.
"... Sailing with the ocean to the right and the shore remaining beyond to the left, Ganges comes into view, and near it the very last land toward the east, Chryse. There is a river near it called the Ganges, and it rises and falls in the same way as the Nile. On its bank is a market-town which has the same name as the river, Ganges. Through this place are brought malabathrum and Gangetic spikenard and pearls, and muslin of the finest sorts, which are called Gangetic. It is said that there are gold-mines near these places. "---The Periplus of the Erythraean Sea (1st century AD). Quoted from The Periplus of the Erythraean Sea, Wilfred H. Schoff, p. 47-8.
"All the country about the mouths of the Ganges is occupied by the Gangaridai with this city : - Gange, the royal residence... 146- 19.15 degree., . "---Ptolemy (2nd century AD). Quoted from Ancient India as Described by Ptolemy, John W. McCrindle, p. 172.
"Next come the wild tribes of the Peukalensians, beyond whom lie the seats of the Gangaridae, worshippers of Bacchus, ... the land here projects into the deep whirling ocean in steep precipices, over which the fowls of heaven in swift flight can hardly wing their way. "---Dionysius Periegetes (3rd century AD). Quoted from The Classical Accounts of Ancient India, p. 423.
Curiously enough we find Gangaridai in Greek myth too. In Apollonius of Rhodes' "Argonautika", Datis, a chieftain, leader of the Gangaridae was in the army of Perses III fought against Aeetes during the Colchian civil war. Colchis was situated in modern day Georgia, on the east of the Black sea. Aeetes was the famous king of Colchia against whom Jason and the Argonauts undertook their expedition in search of the "Golden Fleece". Perses III was the brother of Aeetes and king of the Taurian tribe. Virgil also speaks of the valour of the Gangaridae in his Georgics.
"On the doors will i represent in gold and ivory the battle of the Gangaridae and the arms of our victorious Quirinius. "---Virgil, "Georgics" (III, 27)
It is possible that the Gangaridai people did not only trade with the Greeks/Romans but also fought for them as mercenaries.