can i use a 15 amp GFCI outlet in a 20 amp circuit?
Yes, you can use 15A sockets on 20A circuits, as long as there are 2 or more sockets on the circuit.  That is due to a Code exception, that is made just for your application.
There are 2 other ways you can do the GFCIs, if you are willing to slow up and do some learning about how downline protection works.   (And if you are not willing to learn that, then it’s real simple: Never use the LOAD terminals, not ever, not for anything. LOAD should only be used for downline protection; and the LINE terminals are able to take 2 wires).

You can use one GFCI outlet and one plain outlet out there, if you wire them properly.

You can keep the GFCI device indoors where it is nice and dry and will last 30 years instead of 5.  Then you can feed the entire run to the gazebo from that GFCI, and simply use plain outlets out there.  This also allows 12” direct burial instead of the 24” you normally must do.