The A's need to make room for the return of right-handed starter Rich Harden, who is scheduled to come off the 15-day disabled list to start Sunday's series finale at Texas.
Gaudin has a 3-2 record with a 3.75 ERA in six starts this season after he made 34 starts in 2007 as a full-time starter for the first time.
"It was a very difficult decision with all of our starters throwing well," manager Bob Geren said Wednesday. "The bullpen will be one of our strengths and a deeper one with him. ... When you have five starters throwing well and one of them is not going to start, I don't expect him to be happy about it."
Gaudin, 25, was clearly disappointed when approached following Oakland's 6-5, 10-inning victory over the Baltimore Orioles.
"All I can say is wherever I'm at I'm going to try to pitch my best," he said. "What can I say?"
Harden, on the disabled list with a strained throwing shoulder since April 10, is 1-0 with an 0.82 ERA in two starts this year before he felt some soreness while throwing a side session after Oakland returned from its opening series in Japan.
Harden, 26, was limited to seven games for Oakland last season because of shoulder injuries. He has been on the DL five times in the last three years.
He struck out nine in six scoreless innings in a rehab start for Class-A Stockton on Tuesday, his final minor league tuneup. He allowed three hits, hit three batters and didn't walk a batter.
"Chad's not far from a proven starter," closer Huston Street said. "We know what he can do out of the bullpen in any role. He satisfies an eighth-inning setup guy or someone who can pitch six innings. We're pretty versatile. I know it's going to be an adjustment for him and I know he prefers to start, but with Rich back and him in the pen, it makes us a much stronger team."
Oakland general manager Billy Beane said he would listen to clubs that might be looking to acquire a pitcher, but he also likes having a plethora of pitchers.
"You're always looking for opportunities," Beane said. "I was in the Dominican the last couple of days. I should probably chuckle at it, but it was like, 'They have a big problem on their hands. What are they going to do with this?' You should see my problems a year ago. We've got spots for all our good pitchers is probably the best way of saying it. Ultimately, if there are some needs as we go along that can be addressed by using that, I don't think we'd be against that either."