ShadeEX: I made mine reappear for a while.
Try this:
In Shelf mode hide all your games
Refresh the page (F5)
Then use the account Library refresh link
https://www.gog.com/user/refresh Then unhide your games
This seems to make the game reappear for me whenever it dissapears.
Not shure if it works for anyone else. But feel free to try it out.
EDIT:
Tested and it is still possible to download Fallout even if it doesn't show on your shelf as long as you type in the download link manually.
Not shure if I'm allowed to share the link as a temporary workaround.
But just in case I checked if the link allowed you to download the game without being logged in and it doesn't. But id rather get the go ahead from GOG just in case so I don't get banned.
EDIT 2:
Also I can't check if this allows people who haven't purchased Fallout, but do have a GOG.com account to be able to download it. Wich is also a strong reason why I won't share the link unless specificly allowed by GOG to do so.
This whole post is irrelevant. See post 27 in this thread. Also, the download link does not point towards the actual file and is useless to anyone who isn't logged into an account that has purchased/unlocked the game.