OK, just found it...
The Graduate Performance Diploma is a two-year, graduate-level performance program made up of lessons, recitals, ensembles and one class per semester. It is designed to meet the needs of highly accomplished performers who wish to pursue a more performance-intensive goal than represented by the M.M. or D.M.A. Majors are available in all areas of performance, including opera and chamber music. The Graduate Performance Diploma is considered a full-time program, but since the course load is reduced the tuition is substantially lower than for a degree program.
So I guess for what I want to be doing (hopefully teaching at some college or conservatory), I would get the MM and the DMA, because they have pedagogy in the program, instead of the GPD, which is more for performers.