With the previous, now completed, cancer project, you needed a special config file to use the GPU. But with newest versions of BOINC that's no longer necessary. Unfortunately, as Paul notes, there currently aren't any projects which will make use of a GPU.

I don't see you listed in BOINCstats yet so I don't know what team you're on but if you're on the Ripple Team and want a GPU project, you can try going to their forum and suggesting one of the current projects like Folding@home that makes use of GPUs. I've made that suggestion to them but there would need to be more people interested in that before I would expect them to respond.