It depends on which grandmaster. Not all chess grandmasters play the same.
Stockton played offense in a very straight-laced and conservative fashion. That's fine, there are elite chess players who play like that. But there are also elite chess players who favour more aggressive, slightly ''riskier'' styles and yet are still grandmasters. You can't just say ''oh well Stockton played like a grandmaster'' because there is no one way to do that: you can do it in any number of styles.
While I don't 100% agree with everything that he says, Ben Taylor (author of the book Thinking Basketball, dude who runs the Thinking Basketball YouTube channel, inventor of ScoreVal, PlayVal, and NBA Passer Rating) has them both as top 40 careers of all time and I think he makes some good points about them both in his profiles on the two, which you can read here if you want:
Stockton - https://backpicks.com/2018/01/25/bac...john-stockton/
CP3 - https://backpicks.com/2018/02/19/bac...21-chris-paul/
There's no doubt that Stockton was elite, and when we talk about the top tier of point guards ever we are, to some extent, splitting hairs. I'm not trying to say Stockton was a bad player at all - that is absolutely not the case. I just think that CP3 is better, that's all. In fact I would go as far as to say that CP3 is the best player under 6'3 of all time.