By now you've probably spotted the most obvious hint: the fact that "Greg" starts with "G." It's a little on the nose, but it is certainly worth mentioning.
The strongest hint here, however, is the fact that Greg can transform into a gorilla. You may recall a scene from Ed's story mode in which his squad members were shown. If not, here's the art piece:
That's right. There's a large gorilla character on Ed's team, and as Du Nguyen notes, it has red, green, and blue circles on it. This leads us right to our next clue.
In the new Arcade Edition trailer, what follows G is three spinning circles that contain scenes for Zeku, Abigail, and Cody. The circles shown here are, of course, red, green, and blue.
This could merely be coincidence, but still, it's fascinating.
Obviously, this isn't confirmed in any way. But from what we've seen here, the connection definitely seems plausible.
If G turns out to be Greg, it will be interesting to hear how this came about. Did Capcom purchase the rights to Bloody Roar? Is it just a tribute?