I'm having trouble finding the URL scheme to use... or does this only support signing in with email@gmail.com?
They claim that once it's stable they'll open it up so you'll be able to use "gmail.com" as your identifier in any RP:
http://openid.net/pipermail/general/2008-October/006169.html
Embrace: kinda
Extend: you betcha
Extinguish: ...