GFCI outlet shape for 20 amp vs 15 amp circuit - what does the little line on the outlet mean?
A receptacle without that horizontal slot is only rated to supply 15 amps to a device that's plugged into it. GFCIs are no exception to this. That "20 Amps" stamp is likely stating the receptacle's pass-through rating, or the amount of current it can supply to sockets downstream of it.

There are GFCI devices with that horizontal slot available, which will supply 20 amps to attached devices, and can be found online with a quick Google for "20 amp GFCI receptacle".

You cannot plug a 20 amp device directly into any receptacle without that horizontal bar, be it a regular receptacle, or a GFCI.