U.S. District Judge Charles R. Richey ruled yesterday that the Government Printing Office has been guilty of discriminating against more than 300 women employes because of their sex.

Richey's ruling, which could result in thousands of dollars in back pay for at least 34 of those employes, affects women who work in the section of the huge printing plant in which books are bound. The exact steps that must be taken by GPO to cure the past bias against women will be determined later.

Persons who work as bookbinders in that section are termed "craft" employes and earn more than $11 an hour. Only one woman held such a job there as of last October.

On the other hand, other jobs in that section -- known as Journeyman Bindery Workers -- are considered non-craft jobs and pay about $4 an hour less than bookbinders. Only one of those jobs was held by a male as of last October.

Citing evidence that both groups of workers perform jobs of practically equal difficulty, Richey said the women who work there "have overwhelmingly established that the GPO separate classification system for Bookbinder and JBW (Journeyman Bindery Workers) jobs operates to perpetuate the effects of past discrimination and is not justified for business purposes or for any other reason."