Gonzalez (4-2) settled down after allowing Alex Gordon's homer in the first, holding Kansas City to two runs and four hits. He lasted at least six innings for the sixth time in seven starts.
Hosmer made his first start at first base for Kansas City after hitting .439 at Triple-A Omaha, the highest average of any minor league regular. He went 0 for 2 with two walks.
Sean O'Sullivan (1-2) was almost as good as Gonzalez, but Oakland got to him for three runs in the fifth. The right-hander still managed a career-high eight innings, allowing five hits and one walk.
Hosmer, the third overall selection in the 2008 draft, received a standing ovation before his first at-bat. He walked in his first two plate appearances, stole a base, and took a called third strike with Jeff Francoeur on second with the potential tying run in the sixth. In the ninth, with much of the crowd on its feet, he took a called third strike again.
Grant Balfour pitched a hitless eighth and Brian Fuentes finished for his eighth save in 10 opportunities. With two out, Wilson Betemit singled and pinch-runner Jarrod Dyson stole second. But Matt Treanor grounded out to end the game, sending first baseman Conor Jackson to the ground when appeared to run into his arm.
O'Sullivan retired 11 straight batters before Kurt Suzuki, Ryan Sweeney and Andy LaRoche singled leading off the fifth. LaRoche's hit drove in Suzuki and Mark Ellis followed with a liner to left that glanced off Gordon's outstretched glove. The runners had held up to see if the ball would be caught, so the bases were loaded with nobody out.
Kevin Kouzmanoff grounded to third baseman Betemit, who got a forceout at second as Sweeney scored. Then Coco Crisp's infield out brought in another run, putting the A's on top 3-2.
Crisp was then thrown out trying to steal second, ending the inning with an argument from Oakland manager Bob Geren.
After that, O'Sullivan retired nine in a row before Tim Collins came in.
With two out in the first, Gordon hit Gonzalez's 2-1 pitch over the fence in left. Matt Treanor walked leading off the third and scored on Mike Aviles' triple.
NOTES: Hosmer's mother, father, brother, aunt and uncle flew in from Ft. Lauderdale, Fla., to see his major league debut. ... The A's have not been shut out by Kansas City in 138 games. ... Oakland's David DeJesus has not committed an error in 269 consecutive games, longest active streak among major league outfielders. ... The Royals moved C Jason Kendall from the 15-day to the 60-day DL.