A Compilation of the Games I've Found

• Tremulous (and forks)
  An asymmetric multiplayer game where you can either play as an alien or a soldier hunting the alien. Forks are Tremfusion and Unvanquished, with unvanquished being the most modern of the three. Tactical but not quite in the way I'm looking for

• Wolfenstein:Enemy Territory and ET:Legacy
  ET Legacy is a client for ET, and it brings modern features to it. Both are pretty fast paced and not very tactical.

• Übergame
  It looks pretty good for a libre game, and looks to play good too. About as tactical as ET.

There is https://hypersomnia.xyz/ but it is 2D top down. Pretty tactical though.

A while back I put together a pretty big list of free Linux games, along with their license info. A good amount of them are FPS games, hopefully one of them interests you 🙂

I guess for tactical shooters, Urban Terror might qualify?

Update: I found out that wolfenstein ET was released under the gplv3, so I'll check that out. not quite tactical but I like the game

I’ve heard of something called “Red Eclipse”, which seems to be under CC-BY-SA.

I'm not sure what you mean by tactical shooter but flathub has a subcategory for shooters in the games category. While half of them are source ports of Doom, Quake and some other games, there are a few games I've looked into that seem interesting. So far I've only played Empty Clip and that's a pretty good top down shooter with rpg elements but games like Red Eclipse and Tremulous look pretty interesting as well.

Urban Terror perhaps.

(Not libre) Ready or Not is pretty good