At 9:15 p.m., at the end of a round that lasted nearly six hours, Gal chipped from the greenside rough over a mound to about a foot from the flag. It allowed her to escape with a par and finish a shot ahead of Angela Stanford and Song-Hee Kim.
"I just tried to get it as high up in the air as I could and it just landed perfect," Gal said of her chip on the 18th hole. "It could not have been better."
Stanford birdied the last hole from 30 feet, and Kim made a birdie on No. 18 from 20 feet.
After her late finish, the 24-year-old Gal didn't get carried away with her position.
"You don't win majors in the first round but it's always nice to be on top," she said. "There's a lot of golf to be played and I'm looking forward to it."
Gal won four tournaments playing for the University of Florida before she graduated this year. She finished in a career-best tie for fifth at the Corning Classic in May.
Stanford and Kim were a stroke clear of Yuko Mitsuka and Hee Young Park.
Michelle Wie coped well to shoot 73. Paula Creamer carded 74 without a birdie.
Top-ranked Lorena Ochoa shot 75.
Cristie Kerr had 76, and defending champion Jiyai Shin shot 77.
U.S. SENIOR OPEN: In Carmel, Ind., Greg Norman looked like his old self after Thursday's first round.
There were the signature drives, the string of birdies and those customary big crowds following his every move. He even rekindled an image he'd rather forget: A last-hole struggle.
Norman birdied four consecutive holes on the front nine, but a bogey on the final hole left him in a four-way tie with Joey Sindelar, Dan Forsman and amateur Tim Jackson at 6-under-par 66. Andy Bean and Fulton Allem shot 67s.
Former Augustan Allen Doyle had 70, followed by Augusta natives Larry Mize (73) and Jim Dent (76).
Tom Watson had 71.
Jackson broke the record for low score by an amateur at this tournament, and he did it in his senior debut.
For Norman, it was a chance to show everyone that the man once dubbed the world's best golfer hadn't lost his touch.
"I played well today and the golf course was ready to be had. It was perfect conditions," the 54-year-old Aussie said.
"The greens were just receptive enough to be aggressive with the shots if you want to play 'em."
Norman and those atop the leaderboard made it look easy on the 7,316-yard course, the longest in Senior Open history.