The former Arsenal attacker spent a few months on loan at the Baggies in 2015 with fitness concerns seeing him make just three appearances for the club before being recalled by the Gunners in January.
Unable to secure game time at the Emirates, Gnabry moved to Werder Bremem permanently in 2016 before being signed by Bayern one year later.
Now shining in Germany, the 24-year-old stole the show on Tuesday against Spurs, trolling the home side on social media after the match, with Pulis amazed by the progress he's made since 2015.
"I'm amazed, we had him at West Brom, we took him on loan and we couldn't ever get him fit - I think we even took him off in an Under 21's game," Pulis told Sky Sports .
"He went back to Arsenal, they sold him on, he was a good kid, nice kid, didn't mind him at all and he's really fulfilled his potential.
"You could knock me over with a feather having worked with him at West Brom and seen him there to what he's done is absolutely amazing.
"When people show what they can really do and they really knuckle down and become so good as he's done, it's absolutely fantastic."
Having never previously scored in the Champions League, Gnabry admits his four-goal haul against Tottenham exceeded all expectations and he made a point of getting his hands on the match ball to remember the occasion.
"Maybe I scored four goals in F-Youth. It was definitely a long time ago," Gnabry told DAZN.
"I waited one season for my first goal in the Champions League, but I did not dream of four today, that's something I did not dream of before.
"I saved the ball. It was shot into the fans but I got it back."
Bayern coach Niko Kovac made a note of praising his entire team's performance but conceded Gnabry really impressed.
"First of all I would like to emphasise that this was a great moment for the whole team. But of course, especially for Serge," Kovac told DAZN.
"Really fantastic. He showed today what I expect from him. Not only in the game going forward, because I know what he can do, but also in the game without the ball."