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.

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?

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

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.