The world number seven was in command from the moment he broke serve in the second game.
"I chose to approach the net a lot and that went well for me," Del Potro told reporters.
"I needed to do that against Gael who is definitely on his way back. It won't take him long before he is battling for a top-10 spot again," added the Argentine who was beaten 6-1 6-4 by Roger Federer in last year's final in Rotterdam.
Monfils is on the comeback trail after an injury-plagued campaign last season during which he dropped outside the top-100 for the first time since 2005.
"I'm disappointed. Not only because of the defeat but because I had chances in this match," said the Frenchman. "But overall the body and legs feel great and I know I am improving."
Russian Mikhail Youzhny sustained a back injury against Dutchman Thiemo de Bakker and had to pull out at 4-1 down in the third and final set.