I was using the PGI compiler. I'm not aware of any open source implementation that can automatically take advantage of cuda gpu back when I was still working with fortran-related projects. Not sure about now.
Allegedly GCC support GPU (ptx in particular, for NVIDIA) offloading. I don't know whether the performance is competitive and whether it can be used to speed-up fortran co-arrays (which I, as a non-fortran programmer, would expect to be the way that functionality would be made available to fortran). OpenMP should work as well.
What do you use for the easy GPU processing? PGI? Or is there something else (hopefully open source) that you've found that works?