Have you verified the fuel position on the wing is set correctly in the airplane configuration ?
The configuration for the PMDG 737-800 that comes with GSX is surely correct but, if you made or edited a configuration yourself based on an older version of GSX, your configuration is taking precedence over the GSX internal data, so you must fix the fuel position yourself.
Or, just remove your configuration entirely, and use the default that comes with GSX.