I spent 2 hours downloading 8 gigs and as soon as it is done it just dissappears...how?

More information needed, like how are you downloading and what are you downloading. Are you using the Galaxy client, are you trying to install it with Galaxy, or are you using a web browser to download the standalone offline installer files (to manually install the game yourself), what web browser are you using, what is your OS (Windows 10? Mac? Puppy Linux?) etc.

Anyway, contact GOG support.

also quite importantly which anti virus you are using - maybe it autodeleted the game on finish for no apparent reason

Or maybe it wasn't saved where you are expecting to find it, so do a file search on your computer. gOg typically uses the game name (or some form of it) in the installation file names, so you could search for, say, witcher, and it would find any of your installation files for any games from the The Witcher series.