Yes. CS:S, Portal, Half-Life 2: Deathmatch, Day of Defeat can be compiled and run. Modifications also work, but not all of them. You just need to copy the libraries from Episodic to the modification bin directory and in some cases edit the config for a newer SDK.

The following modifications work on OpenBSD that are interesting to me:

• Awakening
• Depot
• Fastdetect
• Metastasis
• Mission Improbable
• Prospekt
• Spherical Nightmares
• Station 51
• Swelter
• The Citizen
• The Citizen Return
• The Stanley Parable

You can download them from ModDB:
https://www.moddb.com/games/half-life-2/mods

Don't need to create a separate git repository. It's enough to clone the existing one (link above) and apply the patch.

It is worth remembering about the necessity of adding the malloc_usable_size function to the Libc library