g77 supports use of `/*' to start a trailing comment. In the GNU Fortran language, `!' is used for this purpose.
`/*' is not in the GNU Fortran language because the use of `/*' in a program might suggest to some readers that a block, not trailing, comment is started (and thus ended by `*/', not end of line), since that is the meaning of `/*' in C.
Also, such readers might think they can use `//' to start a trailing comment as an alternative to `/*', but `//' already denotes concatenation, and such a “comment” might actually result in a program that compiles without error (though it would likely behave incorrectly).