Gonzalez (16-12) allowed two hits while striking out 11 batters, tying his career-high, and walking three for the A's (74-88), who took two of three in the series.
Andrew Bailey worked a perfect ninth to notch his 24th save of the season.
Alex Liddi and Chris Gimenez picked up hits for the Mariners (67-95), while Ichiro Suzuki went 0-for-3 and finished with 184 hits on the season. It is the first time in his career that he finished a season with less than 200 hits.
Anthony Vasquez (1-6) gave up two runs on four hits in only two innings.
Oakland scored the only runs of the game in the second inning when Sizemore led off the frame with a double and stole third before Miller crushed a Vasquez offering over the wall in right center for an early 2-0 lead.
"We addressed and got better in some areas that were a concern early on," A's manager Bob Melvin said of his team's effort. "I think we really played good defense here down the stretch."
Seattle threatened in the home half when Adam Kennedy and Liddi drew back-to- back two-out walks, but Kyle Seager grounded into a fielder's choice to end the inning.
The Mariners put runners on the corners in the third with two outs, but Justin Smoak flied out to left, keeping the M's off the board.
"He just had great stuff tonight," Mariners manager Eric Wedge said. "We never really got anything going."
Oakland's pitching staff recorded 1,160 strikeouts this season, which is a new Athletics franchise record. The previous high was 1124 in 2009...Gonzalez went 5-1 in six starts in September...Seattle was shut out for the 16th time this season, a new franchise record.