Interestingly, I was one of those "geniuses" that had a perfect ACT score but a horrible GPA (long story). I got rejected by every college I applied to except for the local state school, so that's where I ended up going. In the end I dropped out in my final semester and just went to work, but that's besides the point. I was more than qualified to academically excel at any university I wanted to attend, and I maintained a 3.9 GPA in college up until I dropped out.

The problem with using just GPA is that GPA can easily be manipulated by school authorities who dislike you, test scores cannot.