Australian qualifier Gadecki into first WTA final in Guadalajara
Poland's Frech, seeded fifth, battled back in both sets to beat fourth-seeded Caroline Garcia of France 7-6 (7/4), 7-5.
Sunday's winner in the 500-level hard court tournament will claim a first WTA title.
Gadecki, 22, had never reached a quarter-final until this week, and she hadn't won a tour-level match since January.
At 152 in the world, Gadecki is the third-lowest ranked player to make a WTA final this year, but her run to the final has included victories over former US Open champion Sloane Stephens and second-seeded Danielle Collins.
She is guaranteed to break into the top 100 for the first time when the new rankings are released on Monday.
"This week's been a special one, and to be top 100 is pretty amazing and something I've strived for my whole career," Gadecki said.
"My game plan was really just to play how I've always played. I'm an aggressive player, and I know I do well when I back myself and just commit, and I really feel like I did that well today."
Against Garcia, Frech trailed 3-0 in the opening set and saved two set points before forcing a tiebreak, in which she also trailed 3-0.
She was also down 3-0 in the second, but it was more disappointment for Garcia, whose second-round victory over Ena Shibahara snapped a three-match losing streak that included a first-round exit at the US Open.
Garcia had then reached the semis via a walkover when Marie Bouzkova withdrew because of illness.
Frech, who entered the week at 43rd in the world, is assured of breaking into the top 40 on Monday.