I moved your post in the MSFS section, since you posted it in the FSX/P3D section.
Fact the airplane has an EFB or not doesn't have ANY relationship to the way GSX loads Fuel OR display ( or not ) the GSX Refuel menu, which MIGHT include an option to load the Fuel Quantity indicated on Simbrief, assuming the Simbrief plan has been loaded.
As explained on the manual, and so many times on the forum too, the ONLY thing that controls how GSX Refuels the airplane is the "Show Fuel and Cargo dialog during refueling" option.
- If the Option is Enabled, GSX will refuel the airplane and will show its own Fuel menu, which will eventually contain an option to use SimBrief
- If the Option is Disabled, GSX will NOT refuel the airplane, and will not show ANY Fuel menu, so it won't use SimBrief in any case.
You can't just decide to change the option on airplanes that CANNOT be refuel by GSX: there's a reason why that option is Disabled on some airplanes.
The most notable example is the PMDG 737: you can't just Enable the option so you could add a SimBrief refueling option to an airplane that doesn't have it because, no matter what you do, the PMDG 737 CANNOT be refueled by GSX. That's just an example but, in general, if the airplane has a custom fuel simulation, meaning it doesn't accept being refueled externally, the option MUST be Disabled, no way out of this.