I'd hate to join a server, download any lua it had, take the long process Gmod does to load any map whether I have it or not, to be kicked once I finally got everything.
Wouldn't a section on the G4P forums only for members who have access to the server, with a password in a sticky, be much better than the above?
At the least, to help prevent password sharing, include both.
Password the server.
Then and only then use whitelist.