The White House reshuffled its communications arm yesterday to strengthen a section weakened by the wounding of press secretary James S. Brady and the departure of chief speech writer Kenneth Khachigian.

Named as assistant to the president for communications was David R. Gergen, director of communications in the Ford administration and a former head of the Nixon speech office. Gergen, 39, has been staff director for White House chief of staff James A. Baker III.

Deputy press secretary Larry Speakes, 41, who has conducted the daily White House press briefings since Brady was shot in the assassination attempt on President Reagan March 30, was named deputy assistant to the president and principal deputy press secretary.

Baker also announced that Peter Roussel, 39, director of governmental relations for the Houston Chamber of Commerce, will serve as deputy press secretary for two months beginning July 1. Roussel worked as press secretary for George Bush when the vice president was Republican national chairman and again when he ran for the Senate in Texas.

Another deputy press secretary, Karna Small, will retain her current duties, at least for the immediate future.

The changes brought something of a Ford administration look to the White House press operation. Gergen, Speakes and Roussel, who was assistant to Ford chief of staff Donald Rumsfeld, worked together in the Ford administration.

In his new role, Gergen will be in charge of the press office, the Office of Communications, which deals mostly with news organizations outside Washington, and the speech-writing office.

Speech-writing has been a trouble spot since Khachigian departed May 1. Gergen said yesterday that Anthony Dolan, who has been doing the job since Khachigian left, is on "the short list" of prospects for the job but that no decision has been made.

Speakes, who has been overworked while Brady convalesces at George Washington University Hospital, will share daily briefing duties with Gergen. This announcement touched off a round of questions from reporters about whether a dual-spokesman system would work.

Baker fled from the briefing room while being questioned about this, and Gergen brushed off questions, saying, "Let's give it a try." Then he also departed, turning the daily briefing over to Speakes.

Before they left, Baker and Gergen emphasized that the changes had been carefully checked in a meeting with Brady, whom Baker visited at the hospital Monday. Baker said Brady made one change in the proposed realignment, which was accepted by the White House. Baker would not identify the change.

Reagan has pledged to hold the press secretary job for Brady, who is recovering steadily from a severe head wound, and Baker and Gergen reiterated that pledge yesterday. Gergen said that Brady is receiving more information as he recovers and that a telephone speaker will be installed in his hospital room so he can listen to the daily briefings.

In another matter yesterday, Speakes corrected a misstatement by Reagan at his news conference Tuesday when the president said surface-to-air missiles moved by Syria into Lebanon were "offensive weapons." Speakes said that Reagan was aware that the antiaircraft missiles are "defensive weapons" and that he had misspoken.

Reagan yesterday signed legislation extending the time in which five state medical schools for veterans can use previously allocated federal funds. He said the administration is "doubling the amount" of money being spent on treatment centers for Vietnam veterans but did not mention that he originally proposed to eliminate funding of such centers.