g77 does not support many of the features that
distinguish Fortran 90 (and, now, Fortran 95) from
ANSI FORTRAN 77.
Some Fortran 90 features are supported, because they
make sense to offer even to die-hard users of F77.
For example, many of them codify various ways F77 has
been extended to meet users' needs during its tenure,
g77 might as well offer them as the primary
way to meet those same needs, even if it offers compatibility
with one or more of the ways those needs were met
by other F77 compilers in the industry.
Still, many important F90 features are not supported,
because no attempt has been made to research each and
every feature and assess its viability in
In the meantime, users who need those features must
use Fortran 90 compilers anyway, and the best approach
to adding some F90 features to GNU Fortran might well be
to fund a comprehensive project to create GNU Fortran 95.