Gaining

Ask my gf to feed me?

This is a sticky situation...it's hard to be sure how someone will react when you "come out" as it were. Does she have any inclination as to your desire to gain? If so then that's a positive sign.

In either case it's something that should probably be approached delicately, to avoid information over-load.

Test the water with a few probing questions first.
12 years