Premise:
I've personally tested this method with HL2 Episode 2 + Raising The Bar: Redux - DIV 1 & 2 (which is...AWESOME!). It's pretty unclear how some people does the guides for Linux based systems without considering the pletora of distros and variables we have (mainly due to outdated factors) but all the HL2 MODs can be installed in few simple steps.

Steps:
Pre-requisites for AMD users: AMD opensource drivers (MESA), Vulkan API, Wine and DirectX headers to Vulkan API translation (search in Yast, as usual) and ProtonUp-Qt (flatpak, recommended) then install and configure the Proton GE module version 7.55.

Intel users the same (they share MESA with AMD)
NVIDIA users pretty much the same with the only exception that open source drivers aren't recommended, stick to stable proprietary drivers and don't upgrade frequently due to possible Kernel pairing issues. Best way is to wait subsequent Kernel updates first, and then video drivers later.

1. Install source SDK base 2013 with upcoming option ON under Proprieties > Betas > Beta Participation > (select upcoming)

2. Make it forced compatible using Proton GE 7.55

3. Optionally, run it and do some benchmark and send it to Valve.

4. Copy or unzip the MOD you need inside "sourcemod" folder previously created - Home > username > .local > share > Steam > steamapps > sourcemods

5. Reboot Steam

6. Search for your mod under "uncategorized" list and just add it to your favs like a stand alone game.

7. Play, have fun.

Note for newbies:
Remember, all the folders preceded by the period "." are system folders in Linux. They aren't show up by default just like Windows system or temp folders does. With nice and stable file managers like Thunar or Dolphin you can show/hide them with CTRL+H, simple as that. Even if you have the main HL2 game, or episodes 1 and 2, required for your specific MOD installed into another drive, you always need to copy or unzip directly, the MOD content inside the Steam client path under "sourcemod" folder. Many Linux advanced users, use to separate the "/home" path from the "/" (system) path due to the practical fact that even if a disaster of biblical proportion occurs, they only have to format "/" and reinstall the system in few minutes but without loosing "/home" with all the configurations in it, including desktop themes and HL2 MODs

That's why, gamers and various professionals don't usually like that crappy BTR file system that only eat space with that ridiculous heavy-stuffed snapshots (almost ridiculous like the ties that Black Mesa scientists are forced to wear) and even if openSUSE installers "suggest" that, we, simply don't follow that bad idea. No way.
EXT4 Journaled and XFS Journaled (especially for large amount of graphical stuff according to Silicon Graphics) are still the badasses here.
Badasses, since the glorious 90's.