The person who hosts the database should be trusted.
People who get banned from servers on gmod probably downvote them anyways. No way to prevent that.
The thing about that is that gmod has a pretty big community.
With a small community, just one vote could influence the rating by a great amount.
If someone gets down voted just once the second they host their server, it'd get sent to the bottom of the list.
And there will be those users who down-vote all the servers and up-vote them all, messing up the votes.
It'd be hard to manage.
There is also the problem that Port pointed out.
It's obvious majority of the community would take a bribe and manipulate the votes.
You could however solve this solution by giving access to the User's BL-ID and name that voted for the server.
Maybe even restrain the voting to a single IP and BL-ID too, to prevent mass voting of alts.
My opinion is that if someone took the time to make this right, it'd work out decently.
Giving access to the list of people voting to prove that the rating isn't a lie, and to find users who just down-vote all of the servers.
Maybe even have a way to see all of the servers you've voted on.
Would also be neat to have the ability to leave comments on a server, that way you could find out why a user likes the server.