Wayne Gross, who has a .235 career batting average and is .209 this season, hit a three-run, fourth-inning home run and the Oakland A's rode that and Mike Norris' four-hit pitching to a 4-0 victory over the Kansas City Royals in today's opener of the American League West Division playoff.

Gross' home run followed a two-out error by George Brett, making all three runs unearned, and began the end of Dennis Leonard's dominance of the A's. Leonard had shut out the A's twice in three weeks.

Dwayne Murphy's eighth-inning homer ended a string of 25 1/3 innings Leonard had pitched against the A's without yielding an earned run.

Norris pitched out of bases-loaded trouble twice to gain his third shutout this season and second in a row. Norris won his first six games this year, but finished with a 12-9 record and had been in Manager Billy Martin's doghouse recently.

"It's sad that it takes a Billy Martin to cuss me out to get me going," Norris said, "but that's the kind of guy I am. I need the added awareness. It got results."

Martin threatened Sept. 25 to drop Norris from the playoff rotation. Norris had lost two straight to the White Sox, giving up 15 runs (11-3 and 6-2 scores), including four runs in the eighth inning of a 2-2 game. Moreover, Norris had wrecked his car on the way to one game, and on another occasion arrived at the park at 1 p.m. when he was to pitch at 1:30.

The defeat was the Royals' 29th of the season at home. They won just 19 in Royals Stadium, where they had won more than 60 percent of the games since the stadium opened in 1973.

The Royals brought the only losing record into the playoffs, and counting the loss today are 50-54 this year. They won the second season with a 30-23 record, thus earning two home games in the miniseries against the first-season champion A's.

The A's were 64-45 (27-22 second season) and entered the playoffs with the best record in the American League.

The third through fifth innings represented a microcosm of the Royals' year-long frustration. Kansas City left the bases loaded twice. Brett's pop fly ended a threat in the third inning and Frank White lined into a double play to finish the fifth.

In the third, a walk to Clint Hurdle, a wild throw by Norris on John Wathan's bunt and Willie Wilson's bunt single filled the bases with one out. White hit into a force out at home and Brett flied to shallow center.

In the fifth, Hurdle singled, Wathan walked on four pitches and Washington bunted for a single. Norris had thrown six consecutive balls before Washington's bunt, and the bottom three in the batting order had the bases loaded.

But, Wilson popped up the next pitch, White lined out to Gross at third and Gross threw to Dave McKay to double Wathan at second.

Between the two nonrallies, the Royals suffered the indignity of Gross's home run and Brett's throwing error, plus a stunning collision of Brett and Wilson.

With two out and Murphy on second after a walk and a ground out, Tony Armas hit a bouncer to Brett at third. Brett's throw to first was in the dirt, and Willie Aikens didn't catch it. Gross then hit a 2-1 pitch high to right-center and, boosted by a strong crosswind, the ball cleared the wall beyond the 385-foot sign.

Gross said his homer "may be the only chance I have left to be a hero before they run me out of town.

"Anytime a ball is hit to George Brett, he's got a chance to throw it away. They say that about me, too."

Brett, who left five runners stranded, said, "The fans were on me pretty good. That doesn't bother me. When I start listening to them, it's time to get out of the game. Besides, I probably deserved it."

"Norris was beatable," White said. "He threw slop all afternoon, especially to George. It's hard to respect a pitcher when he never challenges you."

Brett disagreed: "A pitcher should throw what he thinks will get you out. He got me out with offspeed screwballs, and that was good enough. I'd like to see a fast ball, sure, but I'm a good offspeed hitter, too, when I'm swinging good."

Leonard had won six of his seven decisions since Sept. 8, posting a 1.33 ERA, and in the past his record from July to the end of the season has been an average of 11-5. He was 6-7 when the strike came this year, and he finished 13-11. But his string of three consecutive victories was broken, and the Royals' fate will rest Wednesday on lefty Mike Jones, who will become the only rookie to pitch for the Royals in their five postseason playoffs since 1976.

He will be opposed by Steve McCatty, the A's most consistent pitcher this season and a strong possibility for the Cy Young Award.