Adding a standard receptacle to a standard receptacle on a GFCI Circuit
Any sockets placed after a GFCI are protected by the GFCI. So a standard socket will suffice.

One of these toys (make sure it has the GFCI test function) is invaluable. Put it into a standard socket to test the wiring - and then press the button. If that socket is protected, the appropriate GFCI will click off.