Inside the zip archive we can see 2 recently changed files:
1. DEFAULT.CFG
2. DOOM2.WAD

Looking through DEFAULT.CFG file won't give us anything useful.

WAD extention, as you can find out, is id Software's proprietary archive format, for which there are many tools to work with.

So you open that file with any such a tool, for example you can use 'SLADE', and search through for peculiar texture file.

Texture's name is "BRICK"

Flag is "VolgaCTF{D00M_7h3_@w3$0m3_GAME}"