So I wanted to wander around the old lobby 1 map, but at my surprise, there was an awful lot of errors, such as texture not showing, and also disappeared models (fountain, walls and such)
You may not have all of the required games required to show all of the textures correctly. There should be a white controller logo in the main menu or pause menu. Click on it and check Team Fortress 2, Counter Strike: Source, Half Life 2, and both HL2 episode 1 and 2. Load back into Lobby 1 and it should be mostly fine.
I have all the games that GMT needed for textures, and I still get a few errors in the old lobby. I think some textures/models were created by Pixeltail just for GMT, and once they took down most of the workshop content for Tower, those models and textures were lost.