ZDoom

https://store.steampowered.com/app/420740/The_Deed/
Until September 27.

Try to put the soundfont into `~/.config/gzdoom/soundfonts/`

https://store.steampowered.com/app/9288 ... g_Warrior/
Until September 23.

https://store.steampowered.com/app/1448 ... ny_Button/
Until September 17.

https://store.steampowered.com/app/1562780/Brojlery/
Until September 13.

https://store.steampowered.com/app/3506 ... _Survival/
Until August 31.
(Reviews are mostly negative though)

Also you can try some sort of https://flathub.org/ru/apps/com.github.tchx84.Flatseal to grant GZDoom access to folders where wads are.

https://store.steampowered.com/app/2905 ... de_Silver/
Until August 21.

2.) I listen to a lot of emulated video game music and I don't know any music program on Linux that can play it

Try Deadbeef.
(I currently use Foobar2000 + Wine for this stuff, but i used Deadbeef before.)

https://store.steampowered.com/app/2457890/DRACOMATON/
Until August 4.

https://store.steampowered.com/app/2955 ... chen_Wars/
Until August 4.

Edit: As you see, I use two different textures for the locker: one has a stain, and the other doesn't. The stain now is a layer in Photoshop. How do I use layered textures correctly: a) prepare two large textures in a graphical software or b) make layers in UDM? If the latter, how do I create a ...

Should they be a power of two (128, 256, 512)?

The less 1's the binary number has, the faster the computer processes it, so yes, they should.

These are called "3D Floors" in UDMF (https://zdoom.org/wiki/Sector_Set3dFloor)

https://store.steampowered.com/app/1356040/Ampersat/
Until July 6.