Will it be gone for good, or will you be working to get it back?
Good question, I guess it would depend on why the game is being pulled. More then likely Ubisoft's licence to use XII has finally expired and probably felt it was not worth renewing or couldn't renew it, and given that Ubisoft still owns the rights to the game itself as it was made by their Paris division/studio then I wouldn't hold my breath as it would require Ubisoft and whatever parties attached to the XII licence to come some sort of a agreement and a more then likely revenue split between all parties.
Now if Ubisoft decided to sell their rights to the game to whoever(pending of course Whoever has licensed the rights to the XII ip) then GOG would need to ink a new deal with said group/company.
Good thing it was DRM-free. I've got it copied to my external hard-drive for safe keeping.