They ruled India, therefore they are Indian dynasties. That's pretty damn simple and straightforward, and is the accepted meaning of the term. The Yuan and Qing were Chinese dynasties, despite being foreign invaders, and the Norman and Anglo-Saxon dynasties are English, despite being foreign invaders, etc. Don't change the meaning of a term just because you don't have an alternative argument. And the fact that you have yet to make one would seem to indicate that you don't have any other than - "They be foreigners, y'all!"
And the onus is not on me, but on you, since YOU are the one making the claim that the Mauryans - whom I know a little about, not much - and the Guptas - whom I know nothing about - are the greatest Indian dynasties. My sole claim is that the Mughals are greater than the British, which is one I can make as I know the history of both periods. I make no claims as to the relative greatness of these two dynasties to other Indian dynasties. You do. Therefore, it is up to YOU to prove your claim, not me to disprove it. As a qualified, trained historian, you should know this.
Define "truly Indian." Do true Indians commit sexual assaults, or do only untrue ones do so. There's a rather famous fallacy you're introducing there. Quite a few of the dynasties throughout history have been imposed upon a nation from the outside, not from within - I've already mentioned some earlier. Are the Aryans true Indians? What about Pakistanis? Is Sikkim truly Indian?
Good argument about the Guptas being good at science and maths, but there's a lot more to a dynasties greatness than that. Athens was the Mediterranean centre of philosophy, didn't stop it from being defeated and almost destroyed by Sparta. Also, funnily enough, you didn't tell me anything about the Gupta I didn't already know, which considering you're an 'expert' and I'm not, isn't very impressive.
So far as I know it counts as the first dynasty from India to unite the majority of the sub-continent under a single ruler. Asoka was also seen as a great spiritual as well as temporal leader.
I just gave a better argument for something you're supposed to be arguing than you yourself. Give me an argument in favour of the Guptas or Mauryans Bast, not "I like them better." Besides, the Mauryans didn't establish Buddhism, their leader simply converted - after bruatlly subjugating said sub-continent.