G95, as its name implies, is a complete Fortran 95 compiler, and it also has a few F2003 extensions. It has a few warnings about source code that gfortran does not. If a Fortran 95 code does not work with gfortran or (the now free) Intel Fortran, g95 is useful as a 2nd or 3rd opinion. If you have an F95 code I suggest that it should compile without triggering the following g95 warnngs:
100 Unexpected end of format string in format string
112 Variable is set but never used
113 Variable is used but not set
115 Line is being truncated
137 Variable is never used and never set
146 Variable is set and never allocated
147 Variable is used and never allocated
159 Parameter is never used
163 Actual argument does not have an intent
167 PRIVATE module procedure is not invoked
You can compile with g95 -Wall -Wextra -Wimplicit-none -Werror=100,112,113,115,137,146,147,159,163,167 -ftrace=full -fbounds-check -freal=nan -fmodule-private
That said, gfortran is currently maintained, supports F2003, F2008, and much of F2018, and produces faster executables. It is my primary compiler, but I like to have g95 around.