That's because you haven't enabled the "Show the FSX Fuel and Cargo page" setting in the GSX airplane customization page, so GSX assumes this airplane has a custom fuel system that it's not supposed to mess with.
Since that plane is not internally supported by GSX, the only possible reason for that setting to be disabled ( it's enabled by default ), is either you or whoever made that plane custom configuration (maybe it comes with a GSX.CFG file already ?), set it so.