A federal judge here ruled yesterday that the head of the Government Printing Office lacks authority to furlough its 6,000 employes for six days without pay.

Public Printer Danford L. Sawyer Jr. had ordered the furloughs as part of a controversial campaign that he said would save the government millions of dollars. A congressional oversight committee, however, told Sawyer to postpone any furloughs until it had studied the issue, and U.S. District Court Judge Oliver Gasch agreed, ruling that the GPO must obey the congressional committee.

Sawyer, who has battled the Joint Committee on Printing and the GPO's unions in his attacks on policies he considers wasteful, said he was disappointed with yesterday's ruling but would comply with it.

He said there would be responses "at several different levels"--possibly an appeal and probably legislation to strengthen the hand of the public printer.

A millionaire advertising executive who came from Florida to head the GPO in the Reagan administration, Sawyer has enjoyed support on the Hill. Nine senators and 89 House members have cosponsored concurrent resolutions supporting his position, and Sawyer said a joint resolution with the force of law might be introduced in place of the concurrent resolutions.

Sawyer wants legislation that would give the public printer clear authority in personnel matters and make him subject to a board of directors rather than the Joint Committee on Printing.

William J. Boarman, president of the Columbia Typographical Union, said yesterday, "This is a great day for the rule of law . . . . Rather than continuing his propaganda attack on the GPO employes, we hope Sawyer will come to the collective bargaining table."

George E. Lord, chairman of the joint council of unions at the GPO, said the unions would lobby Congress against a joint resolution giving new powers to the public printer, and he said he doubted such legislation would pass.

Sawyer has said the furloughs are necessary because the GPO is losing money and because the amount of printing scheduled has declined this year. He said each furlough would save $600,000.

Union spokesmen said yesterday that for each day the GPO is not working, it loses a $2.5 million payment from Congress. Instead of saving $600,000, each furlough would lose $1.9 million, they said.

Sawyer called that reasoning "black magic economics." "What they're saying is 'just charge it to the taxpayer and don't worry about it.' "