The 777-300 doesn't have a freighter version, so we haven't configured a main cargo door for it.
I'm out of office and I don't have the PMDG 777 installed on my laptop so, I cannot check which variable you should use if you want to use the main cargo door as a bulk cargo door so, for the time being, use the option to ignore it, by setting it as "door with no open/close controls"