20G is a term that refers to pieces moving 20 units per frame, instantly dropping in place. In TGM1 and TGM2 (Master mode), the mode activates at level 500. In T.A. Death and TGM3's Shirase, pieces always move at 20G, while in TGM2's Normal and TGM3's Easy, 20G activates at level 300 and 200 respecitvely. The behavior of this mode closely resembles if one were to hold up in TGM2 and 3.
A piece cannot move across other blocks if the playing field is uneven.
T moved right
One structure that will allow a piece to move either left or right is a pyramid:
With ARS, every piece has an orientation achievable during entry delay that will allow it to land on the tip of the pyramid -- column 5. From this point, the piece can be moved either left or right as necessary. Having column 5 higher or on equal level with surrounding columns helps maximize the number of moves that can be performed with a given tetromino. Still, the player should make an effort to avoid overstacking in the center.
T.A. Death mode
T.A. Death is a mode made famous by the "Tetris Japan Finals" video that has been duplicated dozens of times on YouTube, featuring instantly dropping tetrominoes. The difference between death mode and 20G is un-noticeable until later stages in the game in which ARE starts to decrease and lock delay is very low. Arika currently hosts a video showing a GM grade in Death mode here.