The book Patchy has got seems to have something to do with PuyoPuyo. There is a puyo (puyopuyo) near the top, with the word "rensa" underneath. "Rensa" is the term for the chain combinations that are used in the game, and there is a usage of "GTR" in the game community to refer to a particular technique for creating the chain combos: https://puyonexus.com/wiki/GTR_for_Beginners