Can I use 15 amp switches and outlets with 20 amp service,will be incorporatin gfci

Switches must be rated for the over current deive, so 20 amp circuits should have 20 amp switches.

Using 15 amp receptacles on a 20 amp circuit is fine, since a 15 amp receptcle is different than a 20 amp, you end up limiting the 20 amp circuit to 15 amp becauce only 15 amp plugs will fit into a 15 amp device.