lapack, for example, has USES=fortran[1]. That invokes Uses/fortran.mk[2] and accepts the ports-default fortran compiler, FORTRAN_DEFAULT, which is definedin bsd.default-versions.mk[3] as gfortran (GCC).
[1]: https://svnweb.freebsd.org/ports/head/math/lapack/Makefile?r...
[2]: https://svnweb.freebsd.org/ports/head/Mk/Uses/fortran.mk?rev...
[3]: https://svnweb.freebsd.org/ports/head/Mk/bsd.default-version...