When Punk debuted on WWE television (well, ECW on Syfy) he didn't yet use the famed GTS as his finishing move. Punk instead used the Anaconda Vice, a neck and arm submission move that was set up using a uranagi slam. On the indies, before moving to WWE, Punk used a move called The Pepsi Plunge - a top rope pedigree. The Plunge never made it to WWE TV (for reasons which should be obvious) and Punk adopted the Vice to finish off opponents, going on a few months' long wining streak in ECW. The Anaconda Vice was a decent finisher and usually got a good reaction but Punk definitely needed something more impactful. He debuted the GTS against Hardcore Holly in a winning effort in early 2007. Where did Punk come up with the concept? Easy: he stole it from KENTA, now NXT's Hideo Itami, who was one of the best wrestlers in Japan at the time. Punk was a fan of KENTA and had studied him for years and used the move respectfully. Unfortunately for KENTA/Itami, it doesn't look like he'll be able to use the move on WWE TV if/when he gets called up to the main roster due to the fact that CM Punk chants would definitely ensue, something that WWE doesn't want. https://www.youtube.com/watch?v=am1p7fT-2Qg
