He earned the $217,500 first prize with a stress-free two-putt par at the par-5 18th. The pressure was off because Irwin, who closed with a flawless 66 to tie Gilder in regulation, made a mess of the extra hole to drop his playoff record to 0-4 on the Senior Tour.
Both players started the playoff by driving into the same bunker right of the 18th fairway. After Gilder knocked his second shot out onto a grassy slope, Irwin chose a more lofted club in order to clear the bunker lip.
But the wind caught Irwin's ball and blew it right, and he watched as it bounced high off a cart path and landed in very high grass.
It was all downhill from there for Irwin, who slid his club underneath the ball with his third shot, advancing it just a couple of feet forward. A frustrated Irwin then slashed his fourth shot out of the grass and his ball ran the full width of the fairway before diving into the water guarding the left side.
He dropped his fifth, then hit his sixth shot fairly close to the pin. Gilder played his third shot from a side hill lie onto the green and two-putted from across the putting surface to seal the win.
'Hale got a bad bounce in the playoff and got it up in the hay,' said a sympathetic Gilder. 'I had something like that happen to me in the Western Open. You hate to have that happen in a playoff.'
Irwin, the Senior Tour's all-time victory leader with 34 wins, including three in this event, posted his fourth runner-up finish of the season. He has also won twice in 2002 and remained high atop the season money list with $1,683,306. Doug Tewell is currently second in earnings with $1,235,322.
Sunday's was the first playoff in the 14-year history of the SBC Senior Open.
Irwin was five shots back of Gilder heading into the day but birdied four of the first 11 holes to tie for the lead at 10-under. Gilder, playing in the final pairing four groups behind Irwin, three-putted from 18 feet for bogey at the seventh and was one-over for the day at the turn.
Birdies at the 13th and 17th gave Irwin the clubhouse lead at 12-under 204. Gilder, who got up and down out of a bunker for birdie at the par-five 11th, drained a 15-foot birdie putt at the 16th to draw even with Irwin.
With a chance to avoid extra holes with a birdie at 18 in regulation, Gilder lipped out his 10-foot attempt.
'Best putt I hit all day,' said Gilder, who shot a final-round 71. He seized the lead with a course-record 63 in Saturday's second round.
Gilder moved to 2-0 in career playoffs. The last of his six PGA Tour victories came at the 1983 Phoenix Open after an eight-hole playoff with Johnny Miller, Mark O'Meara and Rex Caldwell.
'I feel pretty good in playoffs,' Gilder admitted. 'I guess I get lucky.'
Bruce Fleisher, a SBC Senior Open runner-up in 1999 and 2000, carded a six- under 66 to grab third place at 10-under-par 206. Australian Rodger Davis (70) finished alone in fourth at minus-nine.
Terry Mauney (71), John Mahaffey (72), Tom Wargo (72) and Dick Mast (72) shared fifth place at eight-under par. First-round leader Ted Goin (70) wound up in a five-way tie for ninth at seven-under 209.
This tournament, a fixture in the Chicago area for 12 years, was hosted for the first time this year by the Port Course at Harborside International, a links-like facility just 15 minutes from downtown. Stonebridge Country Club was the site from 1991 until 1995, followed by Kemper Lakes Golf Club from 1996 to 2001.
Full field scores from the SBC Senior Open