GXX doesn't try to refuel the PMDG 737 by itself because, by default, the airplane is flagged as having a custom fuel system ( "Show Fuel/Cargo page" option DISABLED in the airplane profile ), so GSX is not really refueling, but it's only reacting to the airplane refueling itself.
Of course, if you changed that settings, all bets are off, because the PMDG *cannot* be refueled by GSX.
Another thing entirely is the GSX automation, which will take the fuel from Simbrief and enter in the airplane FMC, which is an "indirect" way for GSX to "refuel" the airplane. Is not really refueling it of course, but it's using the fuel quantity from Simbrief to fill the payload page so, maybe this was the problem you were having ?
If yes, a GSX log for that session would clarify exactly what is happening, because it shows all numbers GSX read from Simbrief and what it has entered in the FMC to debug the problem.