Gaston, who tied with Mitch Jones for the Minor League lead in homers with 35, homered for the second time in Arizona.
The Astros outfield prospect enjoyed a breakout season playing for Class A Advanced Lancaster, hitting .278 while driving in 100 runs. The California League All-Star also smacked a league-leading 15 triples.
"I would love to repeat what I did last year, but last year is in the past, you can't dwell on it," Gaston said. "I would love to have another breakout like last year. Why not me, that's what I say."
Vallejo, playing in front of new Houston manager Brad Mills, collected four hits, but the Saguaros bullpen blew a three-run lead in the eighth inning.
"It was phenomenal," said Gaston of Vallejo's performance. "It was awesome for him. It's really cool to do it in front of your new manager."
Cory Brown (Athletics) hit a game-tying home run in the eighth for the Desert Dogs. Chris Marrero (Nationals) had his fifth straight two-hit game and plated three runs and fellow Washington prospect Danny Espinosa was a perfect 3-for-3 at the plate and scored four times.
Nationals first-rounder Drew Storen (2-0) allowed one hit in a scoreless inning of relief to earn the win for Phoenix (13-8). Josh Perrault (Orioles) picked up his first save after entering the game with the tying run on third and inducing a groundout from Jemile Weeks (Athletics) to end the game.
Josh Judy (Indians) allowed one run on two hits while fanning three over two innings for Peoria (10-11). Vinnie Pestano (Indians) surrendered three runs on two hits in the ninth to take the loss.
Robert Emrich is a contributor to MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.