In a nutshell, GMD had two Xwings. There was a single Xwing in a display case, and one that was part of a display they had at SDCC last year (it was in a long case, chasing a TIE against a fiber-optic star field).
i can't comment on the origins of the Xwing or the TIE that were part of the SDCC display. That is NOT the Xwing being auctioned.
The single Xwing in its case was on display in Apogee's lobby from around 1980 till they shut down in 1993. Grant had it in his office at GMD after that. THIS is the Xwing that is being auctioned. I've heard various tales as to the origins of the parts. There might be some pyro debris in there. Sometimes the charges were duds or they just did a test with one. Talking to Pat, the bulk of the parts were leftover 1976 castings that were in a box that Grant managed to squirrel away before ILM took everything up north. (In other words, the parts were cast up in 1976, and intended for use rather than being cast up 5 or 10 years later for 'fun'.). Some missing parts were replaced with the same kit parts (also left behind). The whole thing was repainted in 1980, hence it looking cleaner than a "Franken-wing" of pyro bits and new castings.
As as to the veracity of the claim that it was screen used, I have my doubts - as do many here. But I do remember seeing this Xwing in Apogee's lobby several times when I visited and interviewed there. (Also saw it at GMD over the years, including at Grant's memorial.). This certainly makes it a period piece with its own story. But then again, I'm not bidding. (It does make me want to get my Capt' Cardboard kit out of the cupboard and finally paint the damn thing.....)
Gene