Adrian Beltre circled the bases on a triple and an error for the game's first run, and Elvis Andrus had a two-run single when Texas scored three more unearned runs in the fifth inning after the second of three errors by Houston.
Gallardo (1-1), who lost at Oakland on opening day, struck out seven in 5 2-3 innings in his first appearance at Globe Life Park, just a few miles from where he grew up in Fort Worth. The right-hander was part of an offseason trade with Milwaukee.
Marwin Gonzalez homered in his first at-bat of the season for Houston.
Roberto Hernandez (0-1) allowed five hits and five runs -- but just one earned -- while walking four in 4 2-3 innings in his debut for Houston. The 10-year veteran right-hander made the team after going to spring training with a minor league deal.
Beltre's first-inning triple was helped by right fielder George Springer, who allowed the ball skip past him when he tried to play a carom off the wall barehanded. The play seemed over when Gonzalez saw Beltre off the bag and tried to surprise him, but he threw wildly past Luis Valbuena, letting Beltre to trot home.
Beltre, who also doubled, had two hits along with Mitch Moreland and Carlos Peguero, making his Texas debut after getting promoted from Triple-A Round Rock.
Peguero filled in for right-fielder Shin-Soo Choo, who is day to day after leaving Friday's home opener early with back spasms.
Gallardo got one of three strikeouts of Chris Carter to leave the bases loaded in the third inning. He retired Jake Marisnick on a popout with runners at second and third in the fourth as the Astros stranded 13 runners.
Gallardo allowed eight of Houston's 11 hits, starting with Gonzalez's homer after he retired the first eight Houston batters.
Carter set up Texas' three-run fifth when he dropped a throw from reliever Joe Thatcher on a two-out grounder back to the mound by Moreland with the bases loaded, allowing Peguero to score. Andrus' bouncing single to left drove in Prince Fielder and Jake Smolinski for a 5-1 lead.
Astros: RHP Josh Fields (right groin) was expected to pitch Saturday night for Triple-A Fresno on a rehab assignment in Reno, Nevada.
Rangers: LHP Derek Holland was placed on the 60-day DL with a left shoulder strain after pitching one inning in the series opener, and LF Ryan Rua went to the 15-day DL after spraining his right ankle in the series opener. ... RHP Jon Edwards was called up from Round Rock.
Astros: LHP Dallas Keuchel starts after throwing seven shutout innings in his first career opening-day start against Cleveland and beating AL Cy Young Award winner Corey Kluber.
Rangers: RHP Colby Lewis, a 3-1 winner over Oakland in his debut, is 5-1 with a 1.54 ERA in 10 appearances with seven starts against the Astros.