Chernobyl L4D2 Campaign maps
It’s a maps from L4D2
original: https://steamcommunity.com/sharedfiles/filedetails/?id=1540128972
usually, maps from Left 4 dead 2 run for a very long time in garry’s mod and often not the first time
so if your map doesn’t start, wait 5-10 minutes or try again
or you can run maps through a third-party directory using mount.cfg