EXXT-GS indicates the electrode has not been approved by the committee for inclusion in A5.20.
The tensile strength has been established and the S indicates it is intended for single pass welds.
I would say that it is not prequalified. EXXT-GS could be for specific applications that may not be appropriate for structural steel due to any number of reasons. The yield point could be anything, it could lack sufficient ductility to pass the required bend tests, it may not pass radiography, or it could meet all the requirements of A5.20, but it is still under consideration and it may be classified in the upcoming revision of A5.20.
You did not state what welding code you are working to, but D1.1 would be one of the few that permits the use of prequalified welding procedures. If that is the case, this electrode is not approved, so you would have to qualifiy the WPS before using it for production welding in the shop or the field.
It looks like the site inspector is on his game and is correct in questioning the use of the electrode your company is providing to their field welders.
Best regards - Al