The opener was a makeup game after Wednesday's contest was rained out.
Gathright batted only once in the contest after replacing Jose Guillen in the outfield in the ninth. KC also got Billy Butler's two-run third-inning double, one RBI from Ryan Shealy and a run on a third-inning wild pitch by Oakland starter Dana Eveland.
David DeJesus had three hits and Miguel Olivo two in the Royals' second straight win.
The winning pitcher was closer Joakim Soria (2-3).
Oakland scored on Jack Cust's two-run fifth-inning double, Bobby Crosby's fifth-inning RBI single and Daric Barton's sixth-inning solo homer. Barton and Kurt Suzuki both had two hits for the Athletics, who suffered their third loss in a row.
The losing pitcher was Joey Devine (4-1).