I say there is a correlation to some degree and it's mostly based on work ethic. Of course you have to have some basal level of intelligence to begin with.
Say you don't give a **** throughout undergrad and you get a 3.0. That's bad but it doesn't necessarily reflect your intelligence if you didn't really try. However, you have to really try to do well on the mcat so if you are a slacker 3.0 student and you study really hard for the mcat you can probably get a good score.
Now the person with the 3.9 gpa probably worked hard throughout undergrad and has a good work ethic. That person will probably also study hard for the mcat and do well. They also probably learned all the material the first time so it will be easier for them.
The only problem would be if you worked hard as an undergrad and still got a 3.0. Then most likely you probably wouldn't score very high even if you studied a lot for the mcat.
This is a generalization.
