Judging from my group of friends, most people who use Facebook do not use Gmail. I'm not sure how well this transfers to the rest of the world, but for almost all of my Facebook friends, none of them will hear of Buzz through Gmail.
I mean, I might be exaggerating, but among my friends I can name at max 10 people who don't have a Gmail and a Facebook. I'm guessing that the true proportion lies somewhere between the extreme situations we're in.
I presume they'll have a standalone Buzz application at the 2.0 or something similar. That will let an established gmail-only social graph generate lots of activity and use, seeming enticing to yahoo and hotmail (and all third-party email account providers) users that can now join/be invited in on the social-web-2.0-sharing fun.