Remove GMTLobbies.zip from the link and then click on GMTTimeCapsule.zip. It will download everything in GMTLobbies.zip and more cool stuff, like Puzzle Impossible and Louie’s Diner (or whatever it was called).
No. You extract and copy the file, go to Computer/Local Disk (C:)/Program Files (x86)/Steam/steamapps/common/GarrysMod/garrysmod/maps, and then paste the file there. Start up GMod, click on Start New Game, click GMod Tower, and there should be more maps there.