Defending champion Williams will face eighth-seeded Polona Hercog in Saturday's final. Williams is aiming for her second straight title on the WTA Tour. She's also won 42 WTA events, the most by an active player.
Hercog defeated Spain's Carla Suarez Navarro 6-3, 7-5 and will be playing in her first WTA final.
Asked about Hercog's game, Williams replied: "I've actually never seen her play. I know only her name."
Their records suggest a mismatch, but clay is Williams weakest surface and in her past two victories, she's been pushed to three sets by little-known players.
"Everybody here against me, they seem to bring their best," Williams said.
Gallovits, ranked only 164 to Williams' No. 5, made few mistakes early, but seemed to lose concentration in the final set when Williams began to hit the corners with serves and groundstrokes.
I'm definitely getting much better, adjusting to the surface," Williams said. "So we'll see. I'm looking to peak tomorrow. Tomorrow is the time."
Hercog, 19, who was also playing in her first tour semifinals, hammered the ball into the stands after the match to celebrate.
"With every match I was playing better and today I felt really comfortable in the warmup," Hercog said. "So I went on the court really confident.
"I had to be more aggressive than she (Suarez Navarro) was, and this really worked out for me. I'm really happy with the performance and to play my first WTA final."
In the men's draw, Juan Carlos Ferrero reached the final when Juan Monaco retired with a rib injury after losing the first set 7-5. Ferrero is seeking his third straight tour title and will face either No. 3 David Ferrer or No. 2 Fernando Gonzalez.
Monaco was leading the opening set 5-4 when he took an injury time-out to treat an injury to the left side of his ribcage. He returned to the court and lost the next three games, handing the set to Spain's Ferrero 7-5, and then retired.
Ferrero has won the past two ATP events on the Latin American swing _ in Argentina and Brazil _ and is seeking his 15th ATP title.
"The streak I am on is really unexpected," Ferrero said. "The way tennis is right now it's very difficult to put together this kind of streak. I'm happy with the wins I am having."
Monaco said he felt the pain when he was leading 3-1 and it got worse, forcing him to seek treatment.
He described it as a rib injury and said it was likely to keep him out of Argentina's Davis Cup tie at Sweden beginning March 3. He didn't know what caused it, and he said he'd never had a similar problem.
Argentina is already without injured David Nalbandian and Juan Martin del Potro.
"I was playing very well and had a good chance to win the match," Monaco said. "It's a shock. The impact is tough for me. Surely I'll miss the Davis Cup."