Pulis quit as Palace manager just two days before the start of this season after keeping them in the top flight last term, and the Welshman marked his first game back at his former club with a long-awaited victory on the road.
Palace went into Saturday's game on the back of four consecutive wins and were big favourites to beat an Albion side who had lost their last three games, but an early strike from James Morrison set the tone for a surprise win.
Gardner then got in on the act with a thumping right-footed drive after the break and there was no way back for Alan Pardew's men.
Three points moves West Brom above Newcastle United into 13th place and deals a blow to Palace's hopes of finishing in the top half.
Joe Ledley was recalled to the Palace starting line-up at the expense of Pape Souare, who dropped to the bench.
Brown Ideye had been a doubt for West Brom due to a knock sustained in training and the striker was only named among the substitutes, with Victor Anichebe replacing him.
Albion had scored just one goal in their previous four away games, but they took less than two minutes to break the deadlock as an unmarked Morrison evaded Jason Puncheon to head home Chris Brunt's corner.
Anichebe was enraged when he went down under a challenge from Mile Jedinak 15 minutes in but referee Jonathan Moss opted not to point to the penalty spot.
Palace started to get into their stride and they also felt aggrieved when Gardner jumped into Wilfried Zaha but once again referee Moss saw nothing untoward.
Yannick Bolasie was this week valued at £20million by Pardew and the winger's price tag may have soared even higher had his acrobatic overhead kick not been palmed away by Boaz Myhill as Palace remained on the front foot.
Pardew reacted to his side's first-half display by replacing Jedinak and Ledley with Dwight Gayle and Souare at the break.
However, the home side were two goals down eight minutes into the second half courtesy of a stunning strike from Gardner.
The midfielder took a touch after a corner had not been properly cleared and then let fly with a sublime right-footed strike from 25 yards which flew past Julian Speroni and found the far corner of the net.
Gayle somehow failed to convert a fine cross from Souare three minutes later and Bolasie felt his goal-bound strike was handled as Palace's frustration mounted.
Substitute Yaya Sanogo had a late goal ruled out for offside for the hosts before Glenn Murray somehow volleyed wide from close range with an effort that summed up a frustrating afternoon for Palace.
