Gary F. Schuster, the reporter who posed as a congressman to demonstrate that security was lax during the signing of the Mideast peace treaty at the White House, was officially repremanded yesterday by a committee of his colleagues.

The action against schuster, Washington bureau chief of the Detroit News, was voted unanimously by the five-member standing committee of correspondents, the governing body of reporters who cover Congress.

Schuster did not attend the hearing in the Senate press gallery, but sent a letter to the committee saying "the piece speaks for my activities that day."

Schuster wrote a front-page story in the News of March 27 telling how he rode a bus reserved for members of the House from the Capitol to the White House and got a closeup view of the historic event. Schuster wrote that "crossing security lines for yesterday afternoon's White House ceremony was a snap. All it took was one call, one question, a little observation and one lie to get a police escorted ride to first class seat on the executive mansion's lawn."

The reporters' committee tookup the affair after the Secret Service and the Capitol Police learned of the Schuster's stunt. The agencies complained to the office of the House sergeant at arms, which passed the complaint on to the committee.

The reporters expressed mixed feelings about the matter. Chairman Michael Posner of Reuters said he "resented" police agencies being "in hot pursuit of a report. . . Within bounds, a reporter should be free to pursue a story."

But Posner said he hoped the reprimand might head off any criminal charges. Committee member Paul Houston of the Los Angeles Times said impersonating a member of Congress could draw imposition of three years in jail and a $1,000 fine. Art Siddon of the Chicago Tribune concluded that Schuster's action "goes beyond ethics . . . I must conclude" that he broke the law.

Capitol Police Chief James M. Powell said last night the matter is closed as far as he's concerned.

In the Police report turned over to the committee, Capt, Richard F. Xander of the Capitol Police said he and Secret Service agent Edward Flynn determined that Schuster had used the name of Rep. Bob Traxter (D-Mich.) when he boarded the bus. Police said Schuster had determined in advance that Traxter would not be there.

In his story, Schuster wrote that he was counting on the fact that most congressman don't know all their 434 other colleagues.

Schuster declined last right to comment on the reprimand.

Reprimands by the committee are rare. A report compiled by Don C. Womack, superintent of the Senate press gallery, listed only 16 other such actions in the last 65 years.