A GNU Fortran installation includes a modified version of the gcc command.
In a non-Fortran installation, gcc recognizes C, C++, and Objective-C source files.
In a GNU Fortran installation, gcc also recognizes Fortran source files and accepts Fortran-specific command-line options, plus some command-line options that are designed to cater to Fortran users but apply to other languages as well.
See Programming Languages Supported by GCC, for information on the way different languages are handled by the GCC compiler (gcc).
Also provided as part of GNU Fortran is the g77 command. The g77 command is designed to make compiling and linking Fortran programs somewhat easier than when using the gcc command for these tasks. It does this by analyzing the command line somewhat and changing it appropriately before submitting it to the gcc command.
Use the -v option with g77 to see what is going on—the first line of output is the invocation of the gcc command.