Do gfci's have to be on a 20 amp circuit, or can they be on a 15A?
The amp rating of the receptacle and circuit do not depend on whether the receptacle is a GFCI or not:


If you have a 15 amp circuit, you must have 15 amp receptacles
If you have a 20 amp circuit, you can either have 20 amp receptacles, or 15 amp receptacles if there is more than one (e.g. a duplex receptacle).


It's not unusual to have a 20 amp GFCI in a residential setting since kitchens are A) required to have GFCIs, and B) required to have 20 amp circuits. You don't generally see 20A receptacles elsewhere in a house since there are virtually no home appliances that require 20 amps @ 110V.