Grealish brilliantly teed up Conor Hourihane for Villa's equaliser and then won a penalty three minutes later which Tammy Abraham converted.
It allows Villa to head to The Hawthorns on Tuesday 2-1 up in their Sky Bet Championship play-off semi-final second leg, live on Sky Sports Football, and Abraham was full of praise for his team-mate.
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Villa went behind when Gayle drilled in from the edge of the area in the 16th minute after Glenn Whelan's error gave him a clear run through.
"Jack got on the ball - and that's what we want from him".
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"I always think to myself does he ever get exhausted?"
But the hosts responded as Conor Hourihane stepped off the substitutes bench to fire home an equaliser in clinical fashion, before Tammy Abraham netted the victor from the penalty spot.
"Even though we had a tough spell in the middle of the season, Jack said just "wait until I get back fit, we will be on the up again".
Meanwhile, Villa boss Dean Smith expects the Baggies to play an open game against them at the Hawthorns.
"We knew it was going to be hard".