Any issue with GFCI outlet touching sides of outlet extender?
Don't take any chances. Give the receptacle a couple of wraps of tape around its perimeter, to cover up the screw terminals for sure. 

The other way to solve a "GFCI in a hard location" problem is to install the GFCI device instead in a location between this one and the service panel... then feed this outlet off the LOAD terminals of the other GFCI.  It can be a recep at an earlier location, a deadfront at a new location (e.g. next to the panel), or a GFCI breaker.