I used a Gatorback belt for the supercharger on the GTP I traded when I bought my Ram. The Gatorback works well because of the mildly abrasive texture to the pulley side of the belt. The cross-grooved surface also seemed to make it last a bit longer.
The down side for the Gatorback is that, in spite of the fact that many web sites call it "The Quiet Belt", I found it to be a hair noisier. But not noisy enough to prevent me from using it.