it sounds like a connection issue then. I've got 3 different GMail accounts set up in my GMail app and none are having issues at all.
Since you're at 100% stock right now, do you have a second Gmail account that you can add? If you do, turn off sync for the first account and then add the second account and let it sync.
If you don't, I have a throw away Gmail account that I created for the fun of it that we can try with and see if it works or not.
Let me know.