[FREE] GOG Wolfenstein: Enemy Territory

must of been free for a while as my son already has it and he often does that claim all free games/dlc thing where you can add a bunch of free stuff at once

First time noticing GOG is more restrictive than Steam.

And funny thing, during checkout GOG tells me this is not compatible with Linux. Yet as far as I remember the game was available on Linux and I even played on it back in the days

It's not a giveaway. It's been released on GOG for the first time and it is and will stay free, just like on Steam when it was released there a few months ago. And GOG even has their own dedicated servers:

https://www.gog.com/en/news/release_wolfenstein_enemy_territory_for_free_with_dedicated_gog_servers_and_bethesda_halloween_sale

There is nothing?

https://www.steamgifts.com/account/settings/referrals

https://www.steamgifts.com/account/settings/referrals

It points from steamgifts (nothing wrong there) to gog.com itself (nothing wrong with that).
There is nothing else in the link!

Steamgifts does this redirecting because of their own referrals.