Looking on the news archive section i´ve found that there are some games that were sold in the past and now are not available. Anyone knows... if one game is going to get off the "sale" list, GOG warns us before?
If you download the game installer you can use it without internet connection? (and save it for furher installations)

Yes, it didn't happen that often (that they remove games) but they always warned us before.
Of course that's why they are called DRM-free.

Historically, GOG has given us a warning as to when a game would be removed from the catalogue, (up to two weeks.) There have been cases when there's been a stealth removal, however.
Yep, no need for an Internet connection when installing a game.

To the first part not sure about a warning.
However if you download the installer you can use it anytime anyplace, without internet connection. I have a external hd with my GoG installers so I can reinstall without having to redownload. Saves a lot of time.

Actually, you can still download the removed games after the deadline; they remain in your collection and all links still work as if nothing had happened. You don't need to back up the installers ahead of time.

Yes, but to the best of my knowledge, all the games that have been pulled from the catalog are still available for download to those who bought them before they were pulled.
EDIT: Ninja'd!

Nice news, thanks!!!!

I don't remember there being warning on Operation Flashpoint GOTY's, Original War's or Obscure's case. Granded in case of OFP pulling was due to conflict between developer and publisher and GOG was most likely just ordered to stop selling it so there was no advance warning. Original War returned about half year or so after being pulled so removal is not always permanent. In Colin McRae Rally 2005's and Toca Race Driver 3's case risk was known in advance and we were given ample warning well in advance on impending removal. It all depends on rights holders.