20A GFCI questions regarding LOAD terminal
First, a GFCI outlet (or the GFCI portion of a GFCI breaker) does not limit current. That's the job of the breaker. 

The functional portion of the receptacle itself is rated 15A (doesn't have the crossbar on the neutral blade slot), but the passthrough is rated for 20A. This is because you can actually use it on a 20A circuit since any device requiring 20A should have a plug that prevents it from being used in a 15A receptacle. 

Here's a 20A receptacle: