What is the difference between 15 amp and 20 amp GFCI?
Also, what is the difference between 15a and 20a GFCI?
By making GFCIs 20A internally but 15A on their face, it allows you to use the same GFCI on both 15A and 20A circuits. If the receptacle had the T-neutral, that would mean it could not be used on 15A circuits. NEC 210.24 allows you to put a 15A receptacle on a 20A circuit if it has 2 or more outlets on a circuit.
Simply so, can you use a 15a GFCI outlet on a 20a circuit?
A 15 amp GFCI receptacle may be supplied power from a 20 amp circuit. Other that gratuitously degrading the performance of the circuit if only one 15 amp receptacle is used in the circuit, there is no reason why it cannot be done. In fact, most receptacles come in pairs with a potential of up to 30 amps.
If your question is “can I connect a receptacle rated for 20 amps to a circuit designed for 15 amps?”, the answer is yes, because the receptacle can handle more current than the circuit is rated for. If you exceed 15 amps, the breaker will blow, and protect the receptacle. The inverse is not true.