How likely is a Steam account ban nowadays?

None

Nowdays they don't really do whole account bans, ie. you will be able to play existing games, but not use community nor buy/add new games.

They don't actively ban those who use VPN to activate games. Only those who buy (even thought another account) using VPN / etc on Steam's own store.

Steam mostly only banns account used in criminal activity like the Russian Key Laundering thing. Also sometimes, foolish people have no-steam installed at launch a game through stream. Those people are only banned for stealing from valve. . .Also credit card Fraud is a big one too. VPNs hmmm not so much.

Only Thing I've heard from VPN is Steam removing games, from time to time.

Accounts are no longer banned...they are simply locked.

You can't get banned because what you did, activating a non-German gift, isn't against the rules of Steam. You can still request to Steam support to switch your game to the German version, in case you are interested... If you bought the game using a trick to use the US Steam store, then you should be worried though. If you bought the game from a US store using a fake USA address, that's a different kind of problem.