This has been a pretty popular (and controversial) subject, but people seem to mean different things by it. Are you referring to the old Lobby 1 map, the player models, or the gamemodes? Because Tower Unite is already much better looking and better optimized than GMT could ever be, as well as having the same gamemodes. TU is a remaster, as well as a successor to GMT.
If you want the old playermodels or maps, consider the Steam Workshop when it's implemented in the future. There will no doubt be old GMT content for players to download made by the community.