Pulis was in charge of the Baggies when Gnabry joined on loan from Arsenal in 2015, with the German making just three appearances before being recalled in January.
The 24-year-old stole the show on Tuesday when he scored four goals in Bayern Munich's 7-2 win over Tottenham.
"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."