Well, those aren't really acronyms as they're directly from GT3.
The LM designation seems to be a 'fantasy' category for cars in the game (apart from the Nissan R390 LM which is an actual car) - from our best guess it seems to be class inspired by a Toyota Supra, Nissan Skyline and Honda NSX that all ran at Le Mans 24 hours in a certain spec (can't remember the details). So, LM stands for Le Mans.
The T1-T9 seems to refer to race tyre spec - again, that is directly from the game.