g77 fails to link the library, complaining that there is no such functions like g6_open__.
This is because of a rather unusual feature of g77, which adds additional second underscore to names of functions which has one or more underscores in the middle. To avoid this problem, use the option -fno-second-underscore when compiling your Fortran programs.
Libraries with date before Sept 26, 2001 contains this bug. If you are using older library, please update it to newer one.