Fortran->C++
C, C++ it is quite easy to translate. Also, quite pointless, as most of Fortran code maintained is in Fortran for a reason.
To GPU - it is more complicated. For large packages it is quite a job. You may give a try. Porting legacy code to GPU is usually a nitpicking task. Not only the code is to be ported to C, all of loop ordering is to be rearranged.
Give us a try if you have a package waiting to be ported to GPU.