What would be the approximate horse power gain by only installing an x-pipe resonator delete on a stock 2015 GT ?

 

I don't know about horsepower gains, but the exhaust will be more free flowing for sure. If you do gain any horsepower, it will be very minimal.
I'm changing out my stock exhaust in a couple of months to a full catback Corsa Extreme with the x-pipe. It's gonna be loud! LOL

 

I would guess somewhere between 0 and 5 HP, mostly in the high RPM range