For additional context on why you might need this, see this post
In short, you can't use mp_gamemode to select gamemode from an addon.
It is handled by the game before addons are loaded and server scripts don't read this cvar.
This plugin does very simple task. It just overrides mp_gamemode value right before level starts. At this point addons are loaded and you can start any custom gamemode without unpacking addons.
mm_gamemode - same thing as mp_gamemode but better
- Put VPK with mutation into left4dead2/addons
- Add +mm_gamemode your_gamemode as srcds_run cli argument
Pre-compiled version is only available for Linux since I don't have Windows system to build but here's source code:
can come in handy with this plugin too.