Fifth seed Cilic recovered from being broken in the first game by his German opponent to win five out of the next six after that, before dodging a break point in the second and converting his second match point for the win and the title.
The win marked Cilic’s fourth Zagreb title and made him the third Croat overall to win at least 10 career titles. Cilic's coach Goran Ivanisevic has 22 titles and Ivan Ljubicic has 10.
His Zagreb record now stands at 22-4, having won 20 of his last 21 matches at the venue.
"It's nice to be in the finals after the tough last year I had," Cilic said. "I played (under) great pressure."
Haas in turn said Marin "played big points better."
Gasquet, as defending champion, came in as favourite against his fellow Frenchman Monfils, who has struggled in title matches throughout his career with a finals record standing at 5-16 even after his second Montpellier crown.
But the fifth seed was assured and confident in his win over Gasquet, not once granting the top seed a break point on his way to a workmanlike straight-sets success.
"This is unbelievable for me," Monfils told reporters.
"I had some back problems at the beginning of the week and I didn't know if I was going to be able to play, so to now have the trophy is pretty crazy.
"I have been elevating my game match after match and I am extremely proud of the way I played today."