Using a GPU to improve compile time

Place and route typically consumes the majority of built time for an FPGA image. This is difficult to do in parallel. In short, it isn't suited for a GPU.

The ideal is a single fast CPU with plenty of cache. If you need to speed up compiles, look at Xenon and AMD based systems. They are slightly (but not greatly) faster than mainstream Intel architecture.

There are several tutorials on using partitions and partial re-compile to speed up builds. If you carefully apply these techniques, they could help speed up builds on mostly finished FPGAs. Note that this is not available in the free version of Quartus.