For fortran, they support the intel compiler. For C++, I believe it's intel on linux and microsoft's compiler on windows. I've seen stuff with people getting gfortran to work, but it requires you to modify the environment variables in the abaqus_v6.env file to accommodate. Google around to see what people have done. It's not necessarily straightforward.
Again, if you don't need to actually use the functionality provided by user subroutines, you don't need the compiler.