Gates' catch capped a remarkable 91-yard drive during which Rivers, in his first year as a starter, threw eight passes in the span of 10 plays, completing five.
"It was a little more than a win," Rivers said. "We grew a lot as a team. We made a big step tonight."
The Chargers know there's a long way to go. But they also know a win over the defending NFL champions can go a long way.
"They're a great team. And you know what? So are we," said defensive end Luis Castillo, who had 1 1/2 sacks.
"Like the coaches said, impose your will on them," guard Kris Dielman said. "I know we did."
San Diego (3-1) got its coach, Marty Schottenheimer, off the hot seat in the process. Schottenheimer, whose conservative approach has long been known as "Martyball," was heavily criticized after sitting on a 13-7 halftime lead a week earlier at Baltimore. The Ravens rallied to win 16-13.
"I can't tell you the last time I lost three games in a row," said Roethlisberger, who threw two more interceptions, giving him seven this season, and was sacked five times. "I feel like I have let my teammates down and let the fans down. I know I can play better and I know I will."
He did look sharp in directing three scoring drives in the first half, but he also was pressured most of the night. He has yet to throw a touchdown pass this season and finished 20-of-31 for 220 yards.
San Diego's go-ahead drive was set up when Roethlisberger threw long to rookie receiver Santonio Holmes on a flea-flicker and was intercepted by Drayton Florence. The Chargers cornerback returned it to the Pittsburgh 38, but Steelers coach Bill Cowher challenged and, after a review, referee Bill Vinovich ruled that Florence had been down by contact at the 9.
LaDainian Tomlinson carried for 1 yard on the first play and took a lateral for 12 yards on the sixth play, but otherwise the Chargers went to the air. Rivers had two big completions to Eric Parker, as well as passes to Tomlinson and Keenan McCardell.
Gates, mostly a forgotten man this season, finished it off when he outmuscled cornerback Deshea Townsend, then dived into the end zone for a 17-13 lead.
"The first half we couldn't run the ball a lick," Schottenheimer said. "That's part of the reason we threw the ball more."
As good as Baltimore's run defense was, "we thought Pittsburgh was even better," Rivers said. "We knew it would be tough sledding on the run, if you just tried to pound them. So we did the opposite."
Rivers was 4-for-7 for 5 yards in the first quarter. He was 20-of-30 for 237 yards in the last three.
"We really didn't have any answer for them in the second half," Cowher said. "He was throwing the ball very accurately. We couldn't get off the field."
Willie Parker scored on a 9-yard run with 35 seconds left in the first quarter and Jeff Reed kicked field goals of 39 and 44 yards in the second period.
San Diego finally woke up after a lethargic start, with Rivers leading an 80-yard scoring drive capped when 6-foot-5 Malcom Floyd outjumped 5-10 Townsend for a 9-yard catch in the left corner of the end zone with 1:11 left.
Rivers was intercepted on San Diego's first drive when Ryan Clark wrestled the ball away from Parker.
San Diego's Nate Kaeding kicked field goals of 28, 33 and 22 yards in the second half.
Tomlinson gained only 36 yards on 13 carries and had eight catches for 34 yards..