Check out the "Opel Web Links" from the "Main Menu" on the left side of this screen. There is a link to the Yahoo Classic Opel Site that has an article about rekeying the GT gas cap. Seems like that is just what you need. HTH
if u need sometrhing to use for a temp gas cap u can use an oil cap off of the gt they fit the same as the gas cap and vise verse so u can have a really cool oil filler cap if u have an extra gt gas cap.