Tournament host Annika Sorenstam, meanwhile, continued to struggle at her namesake event with a 75.
No one could keep up with Gustafson in the third round.
Tied with Webb when the round began, Gustafson started quickly with an eagle and two birdies her first six holes to go four up on the Australian Hall of Famer.
Gustafson continued to grow her lead, bouncing back from a double bogey on the 12th'her first bogey in 33 holes'with two birdies down the stretch. Na-Yeon Choi shot a 68 to tie Webb (73) for second.
Webb was unflappable for two days, with only two bogeys the first 36 holes. She came apart, though, on the back nine of a sweltering round at RiverTowne Country Club.
The disintegration began when she put a ball into a hazard on the par-5 11th, loudly expressing her displease for TV cameras, for one bogey. Things for worse for Webb with three straight bogeys on Nos. 13-15.
On the 15th, Webb stood awkwardly on a patch of gnarly rough amid a bunker and could do nothing else but move her ball a few yards forward into the sand.
When Webb tapped in for that bogey, she was six shots behind Gustafson.
Sorenstam needed a late rally'six birdies in 10 holes'on Friday simply to keep playing on the weekend.
Sorenstam, whos won three times this year, talked of a strong weekend that might get her into contention. Instead, she went backward and was tied for 34th.
The LPGA Hall of Famer has only bettered 70 one time in seven career rounds here.
At 15 strokes behind, Sorenstam might need one of those all-birdie rounds of 54 shes envisioned to reach the top in her last appearance as a full-time competitor at the Ginn Tribute.
I was hoping for a little better, Sorenstam said. Of course, Im bummed out about that. Nothing I can do, but disappointing.
Sorenstam announced her retirement about two weeks ago.
Perhaps Sorenstams countrywoman, Gustafson, can earn the trophy that bears Annikas name for Sweden.
With husband and former LPGA Tour commissioner Ty Votaw trailing, Gustafson took the lead with a birdie on the second hole, then chipped in from behind the green on the par-3 third for an eagle.
A 12-foot birdie on the sixth hole gave Gustafson a four-shot lead. That was still the margin when Gustafson had her lone error by sending her approach on the par-4 12th into a thick, bushy marsh.
Gustafson took a drop, chipped short to about 25 feet and left with her lead cut in half. She got back on track soon enough with consecutive birdies on the 15th and 16th holes.
The way Gustafsons going, she might not want to stop. But Gustafson, who finished second to world No. 1 Lorena Ochoa two weeks ago at the Sybase Classic, wont have to wait long for her chance at a fifth career LPGA win'and first since 2003.
The LPGA Tour moved up tee times several hours Sunday to get the final round in before advancing storms reach the Charleston area.
Ochoa withdrew before play started this week and returned to Mexico because of her uncles death.
Choi, a 20-year-old rookie, moved into contention with her third straight sub-70 round. It was also the second straight day the South Korean made an eagle 2. She accomplished it on the fourth hole Friday and did it on the fifth hole this time.
I had two eagles, but I really had a lot more chances to do it more than twice, Choi said. Thats how great Ive been feeling about my shots lately.
Copyright 2008 Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.