Can a 20 amp gfci receptacle be replaced with a 15 amp gfci receptacle?

Answer 1

Circuit breakers are sized to protect the wires behind the wall from overheating and burning down your house.
If the original electricians put in a 20 amp GFCI breaker, the wires are PROBABLY 12 GA and they should easily handle the 20 amp peak that the breaker will allow.
Those wires will also handle the lower current allowed by a 15 amp breaker.
Simple answer, "yes, you can put a 15 amp GFCI where you previously had 20 amp GFCI"
But the next question is, "why would you want to?" They're within pennies of the same price.
Most electricians will be hesitant to ever install a 15 amp on that circuit. That change will just cause confusion in the future and you're wasting the original efforts that went in to making that a 20 amp circuit.

<><><>

As always, if you are in doubt about what to do, the best advice anyone should give you is to call a licensed electrician to advise what work is needed.

Before you do any work yourself,
on electrical circuits, equipment or appliances,
always use a test meter to ensure the circuit is, in fact, de-energized.

IF YOU ARE NOT ALREADY SURE YOU CAN DO THIS JOB
SAFELY AND COMPETENTLY
REFER THIS WORK TO QUALIFIED PROFESSIONALS.

Answer 2

Yes, you can go down in current protection, but not up. With 15 Amps you just won't have as much capacity to power connected devices. If that isn't a problem you can make the replacement, but turn off the power supplying the GFCI first.