Make sure the game runs at 1920×1080, source games often reset the resolution to 1600×900, when the rift is connected (google how to force a specific resolution for source games).
This makes the image look “messed up” and can prevent vorpx from injecting.
If it still doesn’t work, give the .exe of garry’s mod the name of another source game .exe, like left4dead2.exe or hl2.exe, because garrys mod isn’t supported and renaming unsupported games to supported ones helped me, when vorpx didn’t inject.