Bruce Gelb, whose tenure as director of the U.S. Information Agency has been marked by almost continuous controversy, has accepted a White House offer to become ambassador to Belgium, according to senior administration officials.

Gelb's departure is expected to set off a round of changes in diplomatic posts, with Henry E. Catto Jr., ambassador to Britain, expected to replace Gelb at USIA.

Edward N. Ney, ambassador to Canada, is cited by officials as one of two major candidates to replace Catto. The other, officials said, is a Foreign Service officer, whom they declined to identify.

A senior official yesterday said Gelb's nomination would be announced within days. He must be confirmed by the Senate for the job, which he accepted earlier this month. Over the past year, there have been a series of meetings at the White House aimed at trying to put the agency on an even keel.

Gelb's management style, described as idiosyncratic by many USIA employees, and his disputes with Voice of America Director Richard W. Carlson, have been played out in public for months. Earlier this month, they debated their long-standing feud over the direction and operation of VOA in front of a standing room only audience of 1,000 employees who were instructed by superiors to attend.

Two senior officials said yesterday that Gelb's agreement to leave the post was tied to a demand that Carlson be shifted from his job at VOA. The officials said a new role for Carlson is under consideration at the White House. Neither Carlson nor Gelb would comment yesterday.

Last month, Gelb proposed that VOA's budget authority, personnel and public affairs operations be consolidated with USIA. VOA employees immediately began circulating a petition, which they sent to Congress, asking that they be freed from what they called inappropriate interference by USIA. Senior Republicans on congressional committees that oversee USIA objected to the Gelb plan, complaining that it reversed Reagan administration changes granting VOA more leeway.

Last week, Gelb had his offices virtually torn apart by security agents who had been ordered to look for bugging devices, according to a senior administration official. "We're tired . . . seeing his idiosyncracies on the front page every other day," the official said.

Gelb, 64, is a former vice chairman of the board of directors of Bristol-Myers -- makers of such products as Ban, Bufferin and No Doz -- and an old school friend of Bush. Gelb's friends in the administration complained early in his tenure that Gelb, with no experience in the ways of Washington, was misplaced running the 8,700-employee agency responsible for libraries, educational exchanges and information activities overseas.

Within his first year, he twice tried to fire Carlson. Carlson, for his part, tried to get political pressure applied to have the White House fire Gelb, White House officials said. Carlson, an unsuccessful candidate for mayor of San Diego, was appointed by Reagan in 1986 and reappointed by Bush because of his strong backing from Republicans in Congress and in the administration.

Almost a year ago, the two officials were called to the White House and told to make peace because neither was going to succeed in getting the other fired, officials said then.

Gelb and Bush were prep school classmates and he often has told the story of how their lifelong friendship was ensured when Bush rescued him from bullies when he was 14. He helped raise money for both of Bush's presidential campaigns and sought a high-profile ambassadorial slot as his first choice in the Bush aministration, reportedly seeking a diplomatic appointment to either France or Canada.

Catto, another political appointee, has held a number of high-level government jobs in Republican administrations, including ambassador to El Salvador and chief of protocol at the State Department.