Tetris 99 is very much official guideline Tetris. Arika, the developer of TGM, (and Tetris 99, as pointed out by an adjacent comment,) has done guideline Tetris before, and even implemented it as an option in TGM games in the past (and presumably in TGM 4 which I haven't played yet) and it is assumed this is because of the Tetris Company enforcing it.
You could certainly never do a T-Spin Triple in ARS rotation.
(Though whether it's closer to GM than NES is an interesting question. It's certainly neither since NES Tetris is quite far from what guideline Tetris would become.)
It's my impression that Tetris 99 follows GM more closely than it follows NES.