A quick google reveals people claiming that it's Tom Kane which would explain why you think it's familiar.
Mind you, I have no idea how true that is or where that information is coming from.
It is Tom Kane. He also does that awesome Morgan Freeman impersonation of his on Hoth.
You could have taken the time to see if it was actually listed there; If you did, you would have noticed it doesn't actually specify that Tom Kane voiced Grataa.
People need to stop relying on IMDB like it's gospel or whatever.