The setback, on Tuesday, was the sixth time the former world No 1 has lost his first match of a tournament this year and casts doubt over whether the winner of three Grand Slams can ever recapture his old form.
"It will take a while to get that confidence back - to prove to myself I can beat these guys day in and day out again," he said. "It's just a grind. You've got to pay the price and do it. I certainly am committed to that challenge and I will do it.
"At this stage of the ball game it makes me feel frustrated and like I cannot do it. So it's a tough hump to get over. It doesn't get any easier week after week," he said.
Agassi has returned to competition after a 10-week lay-off because of a right wrist injury, but lost to Doug Flach in Washington DC last week.
"I still kind of kept getting the lead but not remembering how to win," said Agassi, who has dropped to No 32 in the world.
"That should come along. It's like a woman who's been pregnant 10 months," he said. "All of a sudden it just happens."
The 20-year old Gimelstob, a tour newcomer who developed his game at the University of California at Los Angeles, was elated over the biggest victory in his career.
"I had goosebumps when I walked on the court, I was so excited," Gimelstob, who is ranked 103rd in the world, said. "I feel the same way now. To have a win like this in front of my friends and family on my home court feels great."
"Obviously, he's not at the top of his game right now but I have so much respect for him," he said. "But I competed well, played a good match and things went my way."
"I'm not going to say he played the match of his life and I beat him. When he plays the match of his life, you're just praying for rain."
Gimelstob captured the opening set, breaking Agassi in the final game for the third time when he cracked a cross-court winner on his fifth break point to end a 17-point game.
Agassi took a brief lead with an immediate break in the second set, but Gimelstob broke twice to take a 3-1 advantage.
Gimelstob fought off a break point in the seventh game to go ahead 5- 2 before closing out the one hour, 27-minute match when Agassi committed his fourth and fifth double-faults on the final two points of the match.
Agassi was asked if anything positive came out of the defeat. "It's just another match and we'll take the next step from here," he said. "It didn't set me back as much as I didn't break through. It's not going to take one match to win. It's just another match and next time it will be much easier for me to not lose serve after getting up a break." In other matches top seed Goran Ivanisevic, of Croatia, beat the South African Neville Godwin 7-6, 6-3, third seed Mark Philippoussis of Australia hit 20 aces to overcome Brazil's Roberto Jabali 1-6, 6-3, 7-6 and the fourth seed and twice champion, Richard Krajicek, fired 14 aces to defeat Marcos Ondruska of South Africa 6-4, 6-4. Jim Courier, the sixth seed from the United States, overcame South African Grant Stafford 7-5, 7-5.