It's not very clear if with "GSX", you mean GSX or GSX Level 2.
- If GSX Level 2 is not activated, passengers will be simulated only, so they will load very fast.
- If the scenery doesn't have SODE jetways, passengers will be simulated only, so they will load very fast.
- If the scenery has SODE jetways, but you don't dock them, passengers will be simulated only, so they will load very fast too.
The fuel is an entirely different issue. As explained in the manual, an airplane flagged to have its own custom fuel system (and all the PMDG airplanes are), is NOT refueled by GSX. Please look the manual, which has a complete sequence of operation when using such airplanes. The example is made with the PMDG 777, but should be valid for the 747-8 as well.