Gaston, ranked No.155 in the world, booked his place in his first tour-level final with a 3-6 6-3 6-3 victory over Laslo Djere of Serbia on Sunday (AEST).
The 20-year-old had never advanced past the second round at an ATP Tour event before this week. He has now won three matches in a row in a third set.
"I tried to take more time with my game," Gaston said. "I made a lot of mistakes in the first set, so I tried to change my game. My serve was better in the second and third set.
"I am really happy to be in the final. It is going to be a good match, but I will try to enjoy [this] moment."
Ruud, meanwhile, enjoyed a rather more straightforward procession in his semi-final, with world No.249 Vit Kopriva little match for the 22-year-old Norwegian.
The world No.14 needed just 74 minutes to win 6-3 6-0, reaching his second final in as many weeks after his success in Bastad.
Ruud has won 23 clay-court matches already in 2021 and is in the hunt for his fourth career title.