Did you try inputting the key in the site below and it didn't work?
http://thewitcher.com/en/exchange

In that case, you should ask for a refund from support.

No worries. The key was invalidated one day after it was created, but it still remained in my order history.

I prefer to have it on gog. The base game is on sale right now.
Can i use my savegames from steam on the gog version? They are at least a year old.

If they are on the same location (C:\User Profile\Documents\The Witcher 3\gamesaves), then they should work.