I'm going to be talking about Lobby 1 since that defined GMT for me. It was the first map I saw, after all.
GMT was another gamemode. It had a lot of downloads. However I had heard many things about it, so I wanted to try it myself. I had never seen gameplay of it before.
The gamemode itself was really relaxed. There was no set objective, but rather there was a currency system and a map to explore. There was a theatre, a shopping mall, an arcade, some suites, a casino, and some other attractions. Sometimes serverwide events such as sales in the stores or lobby events would occur.
The gameworlds were one of the biggest highlights, though. People queued for gameworlds and played pretty much everything besides Source Karts and sometimes PvP Battle. I played Ballrace the most, and sometimes Minigolf, Source Karts and PvP Battle. I never played Virus nor Ultimate Chimera Hunt nor Zombie Massacre.
Ballrace was far different from what it is now. You'd get only two lives (except on Memories), the UI was goofy, and your character moved slower. The music didn't match a set theme universally, and in general each world was themed after some geographical region (i.e. Sand World, Ice World, Grass World, Sky World, etcetera). Back in GMT, the biggest universal trick was getting Unders, where you hit the goal from below to avoid wasting time getting into the hole.
Minigolf was also different. There was increased friction (as evidenced by the design of some holes in Forest), and the color customization was limited. There were waiting holes where players could practise putting while waiting for everyone to join (More on this later). The graphics were also pretty goofy, and the sound effects were taken from different sources. Like Ballrace, it all worked together.
Source Karts was... boring. The controls were difficult and the game was a little uninteresting. The battle mode was alright though. I'd spend most of my time breaking out of bounds in the one map with the huge vertical ramp however, that was fun.
PvP Battle was cool. After a few games I think I bought every weapon, but only used a select few. Not a whole lot to say, it was fun to play every once in a while.
The suite customization was pretty neat, however I didn't spend too much time in mine. You couldn't hang up pictures from the Internet unfortunately, but you had working televisions then too. There were a lot of features I never thoroughly examined, but you could set doorbells and party music and such.
Now, I never played for the social aspect, and never met many people, but it was still fun to play solo even then. The Ballrace skips and leaderboards were a motivation to keep coming back. Often times I'd forget about GMT for a while and then see it and decide to roam about some. I would play Ballrace and sometimes Minigolf and then run around the Plaza and maybe work towards saving up for a playermodel. In Lobby One, since hats were showcased in your closet, I bought most of them.
Oh, and since it was Gmod there were the typical Source engine things you'd do, like using the console to do things (in Gmod for example, you could do "act parameter" to animate the character). In GMT, speaking of characters, you could purchase playermodels and hats and apply colors to them.
Edit: I forgot to mention that thing I said I'd talk about later, whoops. Join times took forever. Often it would take me until hole 3 or 4 to log in to the game. Always downloading font 7 or something, and my hardware+Internet connection were terrible then as well. Edit 2: The waiting holes. They were just for the players who connected the fastest. They existed in TU for a while but became obsolete with the player ready up system.
Edit 3: In addition to the above, I'd like to also add a note about the achievements. In GMT, when someone earned an achievement, in gameworld or plaza, there would be confetti and a message printed into chat and an audio effect would play. It's incredibly nostalgic and exciting to listen to, and it made earning achievements that much better, even if you weren't the one earning the achievements. The achievement/milestone list in GMT was long, so there were many goals to reach (with some achievements relating to the Plaza map, player actions or movement, or gameworlds typically) and some would give awards in the form of trophy items, equipables, and also GMC I think, depending on the award.
If any of this sounds familiar to Tower Unite, then I suppose it isn't too different from GMT.