Dodgers starter Hyun-Jin Ryu allowed two runs and four hits in four innings.
Gonzalez singled off Jeff Francis to lead off the four-run fifth. Juan Uribe followed with a fly to shallow right. Right fielder Jay Bruce called off second baseman Brandon Phillips but slipped and the ball fell for a single. After Joc Pederson walked, Guerrero homered to left.
Guerrero, the latest Dodgers player signed out of Cuba is trying to learn a new position while keeping his bat sharp. The natural shortstop is moving to second base for Los Angeles.
Manager Don Mattingly has noticed an improvement in Guerrero's offense since the start of spring training.
"He looked stiff at first. It's from not playing in a while," Mattingly said. "I've watched his at bats lately and he looks more fluid and seems more comfortable."
Gonzalez homered off Sam LeCure started another four-run inning in the sixth.
Reds starter Homer Bailey pitched three hitless innings, facing just nine Dodgers batters. Bailey allowed a walk to Guerrero but then got a double play.
The Reds commited two errors in addition to the missed fly ball that was ruled a hit.
"We've made a couple of mistakes that have led to big innings," Reds manager Bryan Price said. "We need to execute better."
Dodgers: Ryu made his second spring start after pitching two scoreless innings against the White Sox on Friday.
Ryu was scheduled to throw three innings or 55 pitches in preparation for the Dodgers' season opening series in Australia in three weeks.
Ryu finished with three strikeouts, all in the third. He walked Todd Frazier but got a double-play ball to finish strong.
"In the first two innings, I didn't have good command," Ryu said through interpreter Charlie Kim. "The last two innings were better. I was working on my slider. I was supposed to throw five and I threw all of them in the last two innings. They were breaking good. I struck out Votto with one."
Reds: Homer Bailey also started for the second time. Bailey signed a six-year, $105 million extension with the Reds on Feb. 20. Bailey, who pitched his second career no-hitter last season, retired the first six Dodgers batters.
Bailey struggled with command in his first start, a two inning effort against Cleveland.
"I'm just trying to build my innings," Bailey said. "It was hard to say if I had better command. I didn't get into any deep counts. They (Dodgers) were aggressive."
Dodgers: Right-hander Zack Greinke threw off the mound for 15 minutes on Wednesday. Greinke strained his right calf on February 28 in his first spring start. ... Non-roster right-hander Ross Stripling had arthroscopic surgery on Wednesday by Dr. Nea El Attrache to remove loose particles from his elbow. Stripling will have reconstructive surgery on the elbow when the swelling subsides. ... Yasiel Puig left early after tweaking a muscle, but Mattingly said he expected him to play tomorrow.
Reds: Right-handers Mike Leake and Mat Latos threw bullpen sessions on Wednesday afternoon. Leake felt tightness in his abdominal muscles from an offseason workout that flared up before his first scheduled spring start on Saturday. He hopes to make his first start on Friday against the White Sox. Latos is testing out the repaired meniscus in his left knee from Feb. 15. Latos also had surgery to remove bone fragments from his elbow in October. He threw 25 pitches. .. Jonathan Broxton also threw a bullpen session of 30 pitches. Broxton, former Dodger, had right elbow surgery on Aug. 23.
VIVE LA FRANCE
Team France, managed by former Dodger closer Eric Gagne, played a team of Dodger minor league players. Team France is on a five game tour of Arizona spring training camps. Gagne was 2003 National League Cy Young Award winner.
Hamilton's bunt single gave the Reds' center fielder a four-game hitting streak. ... Soto has hits in all six of his appearances.
REUNITED DOWN UNDER
"I will get to see Koo. He taught me how to throw a change-up," Ryu said.
Koo, who pitched in 33 games for the New York Mets in 2005, is a member of the Australian national team.
Guerrero's grand slam was the third allowed by Reds pitchers in the first eight games of the spring.