[GTJIM]

Sorry to hear of your GrandMother-in-Laws loss - good to know that she has a man like yourself looking out for her.

The most common way of selling Opel GTs seems to be EBay - have a look by putting "Opel GT" into the search there and you will see that two oe three are usually listed. There is also a Classified Ads section here.

500 miles! Is that the total miles it has ever been driven? If so it does have considerable value as long as it is still in "as new" condition.
The black stripes are intriguing as only the GT/J model had those from the factory - does it have a black "GT/J" decal on the rear panel under Opel letters? If so it is even more unusual.

Hope you can load a couple of pictures up here - worth a 1,000 words!

HTH