With the coming of the Buddha in early 4th century A.D. historical data becomes more reliable. Starting from 4th century A.D. till the latter half of the 6th century AD the facts of the day enable us to come to firm conclusions.
How exactly the Guptas rose to power is difficult to be portrayed. With the collapse of the Indo-Scythian or Kushan empire some tie during the 3rd century A.D. the political pictures for northern Indian began to transformed.
In all likelihood, a large number of independent states must have been formed. The lichachhavis of Vaisali of the days of Buddha re-emerged again. It appears that they obtained possession of Pataliputara and probably ruled as tributaries of the Kushans who had their headquarters at Peshawar.
Very little is known of the early rulers of the Gupta dynasty. The founder of the dynasty was Sri Gupta who bore the titles raja and maharaja. Historically, as can be gleaned from a few Gupta inscriptions, the history of dynasty really starts with king Ghatokacha, the son of Sri Gupta.
Unfortunately, the original borders of the Gupta possession are not known. A number of historians feels that these must have coincided with the borders of Magadha, while others include parts of present day west Bengal as well.
The vagueness of the answers of this question is because of lack of precise epigraphic data. One of the main written sources available is the writing of the Chinese Pilgrim, T-Tsing.