Sounds like your plan backfired to me... Why would you even take a fitness class in college other than to boost your GPA?
While grades are important, they are not everything. You can easily mess with the credibility of your GPA by taking blow off classes. Also, as others have pointed out, GPA means little as time goes on. A 3.7 is just as good as a 4.0. In fact, many recruiters openly state that they would rather hire someone with a 3.5 GPA and outside involvement (clubs, community service, jobs) than someone with a 4.0 and no experience doing anything.
No one is saying that grades mean nothing at all. It's a cost-benefit thing. A 1.0 GPA will get you a job at McDonald's, a 2.0 may get you a better job than that, and so on. However, the closer you get to a 4.0, the less additional benefit you will receive.