g77 strictly assigns types to all constants not documented as “typeless” (typeless constants including '1'Z, for example). Many other Fortran compilers attempt to assign types to typed constants based on their context. This results in hard-to-find bugs, nonportable code, and is not in the spirit (though it strictly follows the letter) of the 77 and 90 standards.
g77 might offer, in a future release, explicit constructs by which a wider variety of typeless constants may be specified, and/or user-requested warnings indicating places where g77 might differ from how other compilers assign types to constants.
See Context-Sensitive Constants, for more information on this issue.