Gilder's 64 tied the course record that J.C. Snead set in 2002. That number was later matched by Hale Irwin, Wayne Levi and Jim Thorpe in 2004.
Dan Pohl and Tom Watson each posted rounds of 6-under-par 66 to share second place. Larry Nelson, the 2003 champion, is one stroke further back at minus-5. He was joined in a tie for fourth place by John Bland, John Jacobs, Hajime Meshiai and D.A. Weibring.
Gilder opened with a 20-foot birdie putt on the par-4 first. He came back two holes later with a birdie at the third. After a pair of pars, Gilder rolled in a 30-foot birdie try on the sixth at Hayfields Country Club.
The 54-year-old made it two in a row with a birdie on the par-4 seventh. Gilder parred eight and nine before catching fire on the back nine. The seven-time winner on the Champions Tour converted a 25-foot birdie attempt on the par-4 10th.
Gilder then birdied No. 11 and made it three straight as his birdie putt on the 12th from 6 feet out found the bottom of the cup. He drained another birdie try on the 14th before parring his final four holes.
'I hit the ball really solid today. I started off shaky, but kept the ball around the hole and gave myself a lot of chances,' said Gilder. 'I made more putts than I have all year. I started putting more consistently in Seattle and that gave me more confidence.'
Watson got off to a quick start with birdies on one and four. He parred his next five holes. The two-time Senior British Open champion dropped in back-to-back birdies from the 10th to move to minus-4.
The 56-year-old took advantage of the 16th and 18th, both par-5s. Watson birdied each of them to get within two of Gilder.
'I didn't drive the ball straight. but fortunately the rough is not very deep,' Watson said. 'I had some good angles into the pins. No bogeys today was good.'
Pohl came out firing as he poured in consecutive birdies from the second. He then birdied the par-5 fifth to get to 3 under. After a bogey on the seventh, Pohl birdied No. 9.
The 50-year-old made it two straight as his birdie putt on the 10th got him to minus-4. Pohl picked up another birdie on the 12th. He posted his final birdie on the 15th before parring out to share second.
John Ross carded the only eagle of the day. That came at the par-5 fifth and helped him shoot a 4-under-par 68. He was joined in a tie for ninth place by Mark James, Andy Bean, Jim Colbert, Curtis Strange and former major league pitcher Rick Rhoden.
Bob Charles matched his age on Friday as he opened a 3-under-par 69. He was joined there by seven other players.
Defending champion Wayne Levi opened with a 1-under-par 71 and stands in a tie for 30th place.