What Red said. GMOD Tower was coded for their servers only, and has stayed that way. It isn't a public gamemode, at least to my knowledge, and seemingly some other people as well. Using that, I doubt CG would implement it as a gamemode type on a new server, seeing as it's openly stealing someone else's work. Not to mention all the haterade, DDoSing, spam, etc that start to be thrown back and forth should something like that happen.