The sixth seed Berrettini capped off a fine week in Switzerland by needing just 59 minutes to dispatch the world number 192 Halys.
"It feels unbelievable. It feels like it was yesterday that I won my first title here six years ago, but a lot of matches and a lot of things happened," said Berrettini.
"I'm just so glad that I can keep playing and enjoying, and I think I found the energy of six years ago during this week. This place is special for me. I'm just so happy," added the 28-year-old who has struggled with injuries since reaching a career-high world number six in May 2022.
Berrettini's second title on clay this season, after winning in Marrakesh in April, will ensure he breaks back into the ATP top 50 on Monday.
Currently ranked 82, Berrettini was outside of the top 150 in March but a return to fitness and a fine 16-6 record for the current season has seen the 2021 Wimbledon finalist begin to refind his best level.
Sunday's final was briefly interrupted by rain just after Berrettini secured a crucial first break in the opening set.
When the players returned 30 minutes later, the Italian then won six of the next seven games to claim his second Gstaad title.
