The 22-year-old qualifier captured his maiden tour-level trophy and became the first Peruvian to triumph on the circuit since Luis Horna at the Chilean Open in Vina del Mar in 2007, but the match was far from smooth sailing.
In a see-sawing final, Buse took treatment for a spell of dizziness early in the clash and trailed 4-0 in the second set which he lost, before racing ahead in the decider to prevent sixth seed Paul from lifting his second title this season.
"It's emotional for my family," Buse said.
"So many people involved that I cannot describe. This is for them. I feel extremely happy. It's the best feeling in my entire life for sure. I feel incredibly happy.
"I'm also really proud of Peru. It's the best country in the world, so I'm just so emotional now."
Paul's loss came shortly after Learner Tien battled from a set down in Geneva to dismantle Argentine Mariano Navone 3-6, 6-3, 7-5 and win his second ATP title after Metz in 2025.
The 20-year-old fourth seed became the youngest American man to win a European clay-court title since his coach Michael Chang lifted the French Open crown in 1989 aged 17.
Fellow American Emma Navarro also warmed up for Roland Garros with a timely confidence boost, winning the Strasbourg crown by beating top-seeded 19-year-old Canadian Victoria Mboko 6-0, 5-7, 6-2 for the third title of her career.
"It's kind of been a little bit of a rocky year and a half or so, but I think we've put in a lot of really good work," said Navarro, who has battled injury and illness this season.
Paul, Tien and Navarro are part of a 37-member contingent of American players - including qualifiers and wildcards - who are competing in the singles draws at the year's second Grand Slam which gets under way in Paris on Sunday.
Buse begins his French Open campaign against Russian Andrey Rublev while Paul meets Australian Rinky Hijikata and Tien faces Chilean Cristian Garin. Navarro plays Indonesia's Janice Tjen in the women's draw.