Can I use a 15 amp outlet on a 20 amp GFCI breaker?

Yes, as long as the circuit feeds more than one receptacle. Table 210.24 and 210.21(B)(3) allow 15 amp rated receptacles on a 20 amp branch circuit specifically because the receptacle is protected by the breaker and does not need to match its rating one to one, this is standard practice and how the vast majority of general purpose receptacle circuits in a home are actually wired. If the circuit serves only a single receptacle, 210.21(B)(1) requires that single receptacle to match the circuit rating, so a lone 15 amp receptacle on a 20 amp circuit that has no other outlets is not compliant.

Why this rule exists

The overcurrent device protects the wire and the connected equipment, not a specific plug. A 15 amp duplex receptacle is not going to be damaged by a 20 amp breaker because no single cord-and-plug device on a general use receptacle circuit is expected to draw the full 20 amps continuously, and the branch circuit conductor, typically 12 AWG, is already sized for the 20 amp breaker regardless of what receptacle style is installed.

Where GFCI adds a wrinkle

Whether the breaker is a standard 20 amp or a 20 amp GFCI breaker does not change this rule, the GFCI function protects against ground faults and operates independently of the receptacle amperage question. A GFCI breaker rated 20 amps still allows downstream 15 amp receptacles under the same multi-outlet circuit logic.

Field takeaway

Mixed 15 amp receptacles on a 20 amp multi-outlet circuit are code compliant and common. The trigger to watch for is a single-receptacle circuit, that is the one case where the receptacle rating has to match the breaker rating exactly.