Can I create a giveaway which is locked to Europe?

i never heard that humble keys have any region lock!

Unfortunately you can't. Just write in the description of the giveaway that the code can be activated only in Europe. I've seen people doing it.
The Resident Evil games are restricted to Europe for sure, I have previously activated Resident Evil 4 DE.

ach wegen der USK.
die sind auf deutschland gelockt.

create a thread and invite people to your whitelist.
just be sure those applying are sure they can activate and play those region-locked games.

Oh another idea: Find a german steam group and make a group giveaway.
This is the official German group (most of the people are from D-A-CH): http://steamcommunity.com/groups/Deutschland-Germany
I am sure you can find some other group youself

Those Keys arent locked to Germany you can activate them everywhere.
But you then get a USK Version (most of the time low violance) KEY which are distributed to German Customers regardless of Age.
Basically our law to protect the Young ones are misunderstood by the world, so even the adult ones of us have to deal with those low violance key thematic.

Off course you should warn People that they will receive a low violance Version of the game.

germany should get rid of USK there is no logical reason for it to exist.
austria has the protection of children and youth also written in its constitution an PEGI works for us just fine.
basically because it does the same thing without censoring games that are already 18+

Germany-only region could be a thing to add. Otherwise it's not Europe only, it is Germany, and rest of the EU gets ROW keys. Deep Silver is the publisher that has Europe as a separate region.