That coin was never issued as a gold proof, so it's either something foreign (i.e. Commonwealth not GB) or a normal 1980 Crown that has been gold plated. I would be inclined to think the latter as I'm not aware of any gold ones from other countries either. It is possible that the Royal Mint struck experiemental gold 1980 crowns. I doubt it, and a solid gold one could be verified in seconds by the wieght. You'll have to post a picture, but I would get any hopes up just yet.