The Reds' Spanish coach Josep Gombau looked to have won the tactical battle with his counterpart Marcelino Garcia Toral as goals in each half from Craig Goodwin and Michael Marrone had the touring team, which finished sixth in Spain's Primera Division this season, on the back foot.
But Gombau's men were rocked back on their heels by second half strikes from Michael Trigueros and Lucian Vietto with Gerard notching the winner in the 90th minute in front of a crowd of 9036.
The Reds were good value for their lead as they surprised the Yellow Submarine with their slick passing game, quick movement and penetration in the final third.
Goodwin fired Adelaide in front in the 33rd minute with a superb angled finish after he had been freed by Spanish striker Sergio Cirio and an unmarked Marrone thumped home a 51st minute header from Goodwin's corner for the second.
Villarreal forced its way back into the match 12 minutes later when the ball broke to Michael Trigueros off keeper Eugene Galekovic in the box and he kept his composure to thrash the ball home.
Vietto's soaring header four minutes from time brought the scores level and then Gerard latched on to a backheel from Vietto on the stroke of full-time to steer home the winner.
The fixture kicked off of a host of games in Australia featuring seven of the world's top football clubs, with Chelsea, Manchester City, Liverpool, Tottenham, Real Madrid and Roma all joining Villarreal Down Under this winter.
