The deal, announced by the Brewers on Thursday, came less than 24 hours after a reported trade of Gomez to the New York Mets fell through because of an undetermined snag.
"When they called me into the office I thought they were going to explain what happened yesterday," said Gomez, who spoke just after news of the trade to the Astros broke. "They said, 'Now you've been traded.' I asked, 'Is this the real one?'"
Gomez said Brewers management, including general manager Doug Melvin, laughed.
For his part, Melvin said he was awake until about 2 a.m., when he told his group to revisit a trade with Houston.
"We were talking to Houston while we were doing the deal with New York, and we re-engaged today," Melvin said. He also said because of the late night, he had so many names spinning in his head that he mistakenly referred to manager Craig Counsell and told people, "We traded Counsell and Fiers to Houston" instead of Gomez.
Gomez, 29, is a two-time All-Star in center field and a 2013 National League Gold Glove Award winner. He's under contract for $9 million next season, so the Astros will have him in their lineup in 2016 before he's eligible to file for free agency. He is hitting .262 with a .751 OPS, eight home runs and 43 RBIs. He has missed time with injuries to his hip and hamstring this season but has played in all but one game since June 23.
"It was tough with Carlos. Very emotional," Melvin said. "To tell him again after what he went through yesterday."
Gomez teared up as he answered questions Thursday, saying, "I'm not shocked by anything after what happened last night."
Fiers, 30, was 5-9 with a 3.89 ERA in 21 starts with Milwaukee this season. Before the Fiers acquisition, Houston's rotation consisted of All-Star starter Dallas Keuchel, Collin McHugh, the recently acquired Scott Kazmir, Scott Feldman and rookie Lance McCullers Jr.
Gomez was listed in the starting lineup for Thursday's game but was pulled about 30 minutes before first pitch.
"It was so close to the game I wasn't even in the clubhouse,'' starter Jimmy Nelson said. "Fiers is a good friend of mine. Gomez is a spark plug to this team.''
Counsell said the traded players will be missed.
"Nobody is ever going to forget Carlos Gomez. He made an impact on the Brewers. I love watching the guy play,'' Counsell said. "Mike has been this under-the-radar guy who has established himself as a major league starter. He never took a back seat to anybody and he had to fight for everything he got.''
The Brewers will receive four prospects: outfielders Brett Phillips and Domingo Santana, left-hander Josh Hader and right-hander Adrian Houser from Houston. The Astros also acquire the 76th international signing slot for the 2015-16 signing period.
Phillips, 21, was a prized piece in the trade for Milwaukee -- so much so that Melvin said he was "supposedly untouchable." He was Houston's sixth-round pick out of Seminole High School in Florida in the 2012 Major League Baseball first-year player draft. In four seasons as an outfielder in Houston's minor league system, Phillips hit .298 with an OPS of .862.
ESPN's Keith Law ranked Phillips as his No. 76 prospect.
Melvin, Counsell and Gomez himself declared Gomez healthy Thursday, one day after a trade to the New York Mets was reportedly nixed at the last minute when the Mets balked because of issues with Gomez's hip.
"In the last month I've played every day," Gomez said Thursday afternoon. "I've missed one game because my manager gave me a day off. I have no problem. I'm playing, and I feel really sexy."
"Our doctors talked with their doctors," Melvin said of talks with the Astros. "They were fine with it."
When asked if the situation highlighted the notion that two teams could read the same report differently, Melvin responded: "You could say that. But it's happened before. There shouldn't be anything directed at the Mets. Everyone has to make those decisions."
He said not to expect his team to sit still after the non-waiver trade deadline on Friday. "I've made a lot of pretty good deals in my career after July 31st,'' he said. "We acquired (Jonathan) Broxton after the deadline last year.''
Agent Scott Boras regarded the Astros' decision to trade for Gomez a day after the Mets' trade fell through as validation that Gomez is healthy and that his hip is not a concern.
"Milwaukee medical experts and reports were viewed as such by Texas doctors,'' Boras said in a text message to ESPN.com. "Apparently a few New York doctors viewed the reports as 'cheesehead medicine.'"
Mets GM Sandy Alderson reiterated Thursday that the Mets believed Gomez's health was too big a risk.
"Our doctors thought the health risk was too great; Houston's doctors apparently feel otherwise," Alderson said. "End of story."
The Astros entered Thursday at 57-45, holding a one-game lead over the Los Angeles Angels in the American League West. They were reportedly involved in trade talks for Cole Hamels, who was dealt to the Texas Rangers; Chicago's Jeff Samardzija; and San Diego's Tyson Ross before making the deal for Gomez and Fiers.
The Associated Press contributed to this report.