Check if the gnu fortran compiler has a -g77 option.
1.3 GNU Fortran and G77
The GNU Fortran compiler is the successor to `g77', the Fortran 77
front end included in GCC prior to version 4. It is an entirely new
program that has been designed to provide Fortran 95 support and
extensibility for future Fortran language standards, as well as
providing backwards compatibility for Fortran 77 and nearly all of the
GNU language extensions supported by `g77'.
The GNU Fortran compiler implements ISO/IEC 1539:1997 (Fortran 95). As
such, it can also compile essentially all standard-compliant Fortran 90
and Fortran 77 programs. It also supports the ISO/IEC TR-15581
enhancements to allocatable arrays, and the OpenMP Application Program
Interface v2.5 (http://www.openmp.org/drupal/mp-documents/spec25.pdf)
Specify the standard to which the program is expected to conform,
which may be one of `f95', `f2003', `gnu', or `legacy'. The
default value for STD is `gnu', which specifies a superset of the
Fortran 95 standard that includes all of the extensions supported
by GNU Fortran, although warnings will be given for obsolete
extensions not recommended for use in new code. The `legacy' value
is equivalent but without the warnings for obsolete extensions,
and may be useful for old non-standard programs. The `f95' and
`f2003' values specify strict conformance to the Fortran 95 and
Fortran 2003 standards, respectively; errors are given for all
extensions beyond the relevant language standard, and warnings are
given for the Fortran 77 features that are permitted but
obsolescent in later standards.
Another user posting on this site keeps posting with problems installing an older gnu fortran verion. I don't know if that is even necessary. The documentation for gfortran seems to indicate that it isn't, but sometimes the devil is in the details.
PS: I still remember buying keypunch cards in the college bookstore. So it's been I while since I programmed in Fortran or used a fortran program like pcap.