The scores were 7-6 (7-5), 6-4.
The unseeded Grosjean is the sixth Frenchman to win the Lyon title.
He rose to 4-9 in his career in ATP finals.
Grosjean has fallen to No. 81 in the latest world rankings after being 28th when the season started.
"He was too solid, but I am happy with my week,” Gicquel said.
The 30-year-old Gicquel, who also was not seeded, remains without a title in his career.
In 2005, he was a quarterfinalist at this event.