Gay, a 47-year-old journeyman whose most recent of four victories on the PGA Tour came in 2013, fired a seven-under-par 64 on the Monterey Peninsula Country Club course to move to to the top of the leaderboard.
Langley, who is yet to claim a win on the Tour, reeled off nine birdies and two bogeys on the Monterey course on his way to a share of the lead.
His round included a birdie on the par-three ninth, his 18th hole of the day, to leave him alongside Gay on seven under.
Five-time Major winner Phil Mickelson was among a group of six players tied for third place on six under which also included Australia's Jason Day.
Mickelson thrust himself into contention after hitting every fairway for the first time in his career.
"I may have done it before, but I don't ever recall doing it," a satisfied Mickelson said afterwards.
The tournament, which sees professionals play alongside celebrities, is spread across three courses near the picturesque town of Carmel -- Pebble Beach Golf Links, Spyglass Hill Golf Course and Monterey Peninsula.
Mickelson started slowly with two birdies and a bogey on the front nine before erupting down the stretch with five birdies and four pars.
He carved out birdie chances with some scintillating iron play, most notably with two solid approaches on the 12th and 13th holes to set up birdie putts which he duly drained.
"It was a really solid round, good solid start, and we had beautiful weather and the golf course here is in spectacular shape," said Mickelson, the world number 29.
If history is any guide, Mickelson will draw encouragement from his opening round. On the three previous occasions he opened with a score of 65 or better at Pebble Beach, he went on to win.
He is chasing a fifth victory in the tournament after missing the cut at the Phoenix Open last week.
Grace got off to a flying start with a birdie on the first followed by an eagle on the par-five second, when he guided his second shot off the fairway 237 yards to leave himself a 24-foot putt.