Tsonga hit 19 aces including two to close out the match in 1 hour, 31 minutes at Ariake Colosseum.
Latvian Gulbis had 11 aces and looked set for an upset but Tsonga took control when he broke the 98th-ranked player to win the second set.
"It's difficult when a guy is serving well like that," Tsonga said. "I just tried to stay focused. I had a break at the end of the second set which was good for me because he was a bit nervous after that."
Tsonga's semifinal opponent will be French compatriot Gael Monfils, who downed Switzerland's Stanislas Wawrinka 2-6, 6-3, 6-4.
Eighth-seeded Lleyton Hewitt of Australia eased into the semi-finals with a 6-4, 6-0 win over France's Edouard Roger-Vasselin.
"Today the whole second set I served extremely well which made life easier," said Hewitt, who broke serve five times from 12 opportunities in the 67-minute match.
Hewitt, who won here in 2001, will face Russia's Mikhail Youzhny in the semi-finals on Saturday. Youzhny knocked out defending champion and fifth-seeded Tomas Berdych of the Czech Republic 6-2, 7-6 (6).
Youzhny lost narrowly to Berdych in May and said he was determined not to let that happen again.
"It's always close when we play," Youzhny said. "I lost to him by one point in Munich so I didn't want that to happen again."
Top-ranked Roger Federer pulled out of the $1.2 million event, citing fatigue, along with third-ranked Andy Murray of Britain, who withdrew because of a wrist injury.