gfortran is free software (part of GCC, the GNU Compiler Collection) and fully implements the Fortran 95 standard and has support for many features of Fortran 2003 and Fortran 2008 standards as well. It's definitely an option you should consider. 1
ifort is actually available on Linux and, in some situations, free of charge. For example, it's free for students and classroom educators. It's free for Open Source developers (you qualify if you have projects on GitHub, and agree to a non-commercial license) and otherwise you can also do a 30-day trial of the commercial offering. 2
You might also want to look at flang, the Fortran compiler that is part of the LLVM project. 3
I believe both will allow you to access external subroutines and modules. You mentioned "front-end", do you mean you have an application that uses a GUI Toolkit, like a Desktop application? If that's the case, it's possible that the GUI library it uses is available on Intel Fortran only, or even on Windows only (in case it's the Windows GUI API), but it's hard to tell without more details on what exactly you're trying to compile...
My suggestion is that you try the compilers above and see how far they'll go. If you still get stumped (for example, building the parts that depend on a GUI API), then post specific questions about those. You might get more useful help that way. Good luck!