I used to be able to develop on my mapping projects with the Steam version, however I can only open a few BMS maps without crashing on singleplayer, and when I do they are severely damaged. This has only occured since recently. Has something caused for these maps to not be compatible for mounting and use in Garry’s Mod?
There have been many, many engine changes since the Steam version, so it is not surprising that the maps are no longer interoperable. You will at the very least need to recompile the maps using a Garry’s Mod-specific set of development tools.
There are many BMS-specific entities, and trying to load them in a different environment will result in a crash.
Aside from that, you need to make sure you have all the assets either packed with the map or pointed to by whichever game you’re using (in this case: Garry’s Mod, and the file where you point stuff is: gameinfo.txt).
Just like Admiral Sakai said, at the very least you want to first recompile the maps yourself; you can find the map sources in the /mapsrc/ folder.