>>271819305
HL2 levels can be loaded through the VR mod for GMod.
https://steamcommunity.com/sharedfiles/filedetails/?id=1678408548
For anything that doesn't have a mod, you're stuck with Vorpx, which is expensive and usually doesn't work well.
>>271819669
Bigscreen is just a virtual screen, but if that's what you're looking for, it should be fine too.