Gonzalez came into the game having allowed three runs in his first 30 Major League innings.
He left after allowing six in only 5 2/3, with the sixth inning serving as his undoing. Gonzalez also surrendered eight hits and had one walk and one strikeout.
"He just wasn't as sharp with his location and with his pitches as he has been," Rangers manager Jeff Banister said.
Gonzalez walked Ben Zobrist to lead off the sixth and hit Billy Butler to put two on with one out. He then allowed an RBI double to Ike Davis, but struck out Brett Lawrie to put Phegley in a pressure situation.
Phegley -- who also homered in the fifth -- delivered, and after Texas reliever Keone Kela gave up a double to Marcus Semien, Gonzalez was charged with six runs and took the loss.
Gonzalez attributed his struggles to a lack of command. After a clean first two innings, Gonzalez had trouble with his two-seam cutting rather than sinking the way he wanted, and he started falling behind in counts.
"I started nitpicking and not really attacking the glove, trying to go around it," Gonzalez said. "Just trying to hit the plate. But yeah, I've got to attack more hitters in those situations and hopefully they get themselves out."
The loss was Gonzalez's first bad outing in the Majors. He entered Tuesday with a 0.90 ERA in four starts, the lowest through any pitcher's first four starts since Atlanta's Kyle Davies had a 0.77 mark in 2005.
"Yeah, it was a grind," Gonzalez said. "I struggled, as you could tell."
Suddenly, the narrative surrounding Gonzalez is no longer how long he can keep his run of success going. It becomes how he can respond now that the run is over.
Gonzalez, though, has a simple solution.
"Don't repeat that," he said. "Just look at my success and not reflect on the negative as much."