The Frenchman, who had earlier secured his place in the ATP World Tour Finals, came through 6-3 6-2.
Both players served well in a tight first set, and while Nishikori was able to save one break point at 4-3, he could not stop Gasquet from taking the game and subsequently taking the set.
In the second set, Nishikori's fight faltered and Gasquet started to put pressure on his opponent with some excellent serve-returns.
Gasquet then found some extra confidence, delighting the home crowd with his full repertoire of shots and wrapping up the win.
The win was Gasquet's 50th of the season, a record-high for the world number 10.