Support are just telling me to install the game and let them know what error I get. I have put hundreds of hours into the game and have lost it all because I got a new PC. My other games are ready to download but this one takes me to the store page to buy it - does anyone know how I can get it back?

The ONLY reason GoG revokes a game is if they discover that it was purchased with a stolen credit card. If your copy was absolutely NOT purchased with a stolen card, you will be able to find it listed on this page:

https://www.gog.com/account/settings/orders

Send the order number, date, game name, version and etc. to Support and they'll fix it. Personally, I've never had a game come up missing, but that's how you do it.