Perfect solution. Latest gsx update now offers to use simbrief data directly and not need loading fenix.
Something's wrong here, it seems you must have enabled the "Show MSFS Fuel/Cargo page" option in your custom airplane configuration. This is Disabled by default, and it should stay that way, because it means GSX will try to actually refuel the airplane, which it simply can't, since it has a custom fuel simulation that GSX cannot interfere with.
That's why you are seeing the Simbrief option and the default GSX fuel menu, but it's not supposed to appear if that airplane is configured correctly, that is with the above option Disabled as we set it by default, so GSX won't try to refuel the airplane, but it will just adapt its own animation to the airplane refueling itself, using the "Load aircraft" button in the Fenix EFB, which is the one and only way you can refuel the Fenix.