This really caught me off guard when I got my Palm Pre in '09. Thankfully I had not gotten into the bad habit of accepting friends on FB or other things for games or anything else. My brother on the other hand....he ended up deleting like 700 Mafia Wars friends so his phone would be cleaner.
This is Synergy - and it really is, even today, ahead of its time (even though its over 2 years old now). Think about it. You never have to make the announcement that you lost your phone and need people's numbers. Or need to input someones new email address. If they change in on FB, then viola, it's automatically updated on your devices.
My response isn't much help on how to limit it to gmail only, I know. What I'm trying to encourage you to do is harness this ability, don't shy away from it. I actually don't know how to limit it to gmail because my contacts are all cleaned up and linked up.
I suggust 2 things:
1. clean up you FB and AIM friends list.
2. go through your contacts and begin linking accounts. You only have to do it once (if webOS hasn't done it for you already)