The midfielder starred against his former club as the Gills won 2-0 at Priestfield on Saturday to continue their encouraging start to the season.
Stimson told the Kent Messenger: "He has got great potential to play higher than League One. He is still young, still learning, there are lots of areas in his game to improve on and he knows that. He has tremendous pace and strikes the ball well."
He added: "I sensed Curtis was up for the game during the week because he trained the best he has since he came here.
"He is a player who can take it easy at times, and get away with it, because he is such a good athlete and a good player. If he pushes himself every day, we will see more performances from him like that.
"Millwall and Leeds are two clubs where he has been. He did well at one, not so well at the other. If I get the same reaction at Elland Road next week, I will be delighted."