jr conlin's ink stained banana

Ok, so let's get a few things clear.

1) this is not universal free wifi. In fact, all it says that google will be setting up some open access points in downtown S.F. Oh yeah, and you REALLY want to run some VPN if you use them since every bad guy in S.F. will be snooping these.
2) They offer you a free VPN, which routes your information through Google, which they intern will use to send even more targeted ads to you, specifically, based on your habits and browsing choices.

Let's note a few other things, shall we?

1) Bad Guys will probably not be snooping the link. Instead they'll load up a step van full of wireless bots and go camp around various plotted access points. Why infect boxes when you can just have them run for real? Good luck trying to connect while these buggers are going full stream.
2) If, by some insane stroke of luck, you are able to connect to one of the open access points and are able to get any form of actual data throughput, there's nothing stopping you from running any other VPN, Tor client, or anything else you like. Personally, i prefer running SSH and proxying data off of my back-end server.
3) Yes you can run Google's VPN off of any connection. It's the joy of a VPN. i'll remind you that Google's sole revenue source is by selling ads. Ads are most effective when they collect as much information as they can about you, file, sort and demography you, and track your choices and actions. i'm betting that less free, but less tracked T*Mobile account is looking better and better.
4) Likewise, there's nothing stopping you from running your own proxy server and tunneling to the connection via SOCKS/SSH. Heck, there are plenty of places that you can connect to if you don't want to do that (naturally, you takes your chances with those).
5) Or you can follow the sage advice of never presuming that an open connection is secure, so you don't do things like log into your bank, pay your credit card bill, check your email or do anything else where your password may be exposed. Go look up local restaurants, Go check movie times, heck, go surf for porn, just don't do anything where you don't feel comfortable putting your information up for public display.

And yeah, i'm betting that some restriction like owning a gmail account will not block big abusers since those are cheap to acquire, and the smart bad guys have been harvesting distributed invites so that you can't reliably kill the tree of ownership.

Good God, it's like the word "google" has some weird Pavlovian response with people.