Gamez's back-to-back birdies moved him to 18-under 198, one shot ahead of Justin Leonard and D.J. Trahan heading into the weekend at the PGA TOUR's only five-round tournament.
Leonard also shot a 67 on Friday, while Trahan had a 68 to finish at 17-under 199.
Boo Weekley fired a 10-under 62 -- one of two 62s on the day -- to move from a tie for 34th place overnight into solo fourth. He was at 15-under 201.
Kenny Perry (65), Anthony Kim (67), Tim Petrovic (68) and Steve Elkington (69) were tied for fifth place at 13-under 203. Chez Reavie (67), Ben Crane (69) and Brett Rumford (69) stood another shot further back at 204.
Battling for the lead with Leonard and Trahan for much of the round, Gamez missed a 3-foot par putt at his 16th hole -- No. 7 at PGA West -- to fall one shot behind the clubhouse leaders.
But he knocked his approach to 3 feet at the eighth to set up birdie, then rolled in a 15-foot putt for another birdie at the ninth to take the lead.
They were his sixth and seventh birdies at PGA West, the shortest of the four courses in rotation this week.
'That's what you do when you play well,' Gamez, playing on a sponsor's exemption, said of bouncing back. 'I didn't let it get me down. I made a bogey -- it happens. I knew I was playing well and I could make a couple birdies coming in.'
Leonard also played PGA West on Friday, picking up five birdies in a flawless round that left the 2005 champion where he was overnight: one shot off the lead (Gamez and Trahan shared it).
'I had some chances today, but didn't really take advantage of them the way I have the rest of the week,' said Leonard, who shot a 64 Thursday at La Quinta. 'I need to make some more putts this weekend, but [winning is] doable.'
Leonard carded his last birdie at his 15th hole -- No. 6 -- to tie Gamez and Trahan for the lead. It was one of the many deadlocks Friday on what has been a tight leaderboard since Wednesday's first round.
Trahan, steadier with the putter than he has ever been, gained sole possession of the lead with a birdie on the 16th at La Quinta. But he followed that with a bogey at the 17th to drop back to 17-under.
After posting a 64 Thursday at SilverRock, Trahan made seven birdies and three bogeys Friday -- more errors than his other two rounds combined. But he's in good position heading to the weekend at a tournament where he missed his first two cuts.
'There's so many birdies being made, if you shoot 3 under you feel like you're getting left in the dust,' said Trahan. 'Even at 4 under for me today I feel like I'm going to lose a few (shots).'
Scott Verplank shot a 70 and led a group of four players who were tied for 12th place at 11-under 205. He was joined by Shane Bertsch (64), Stewart Cink (66) and John Merrick (67).
The Classic Course is the fourth in the rotation and will host the final round.