Rep. Gillis Long (D-La.), a member of a famous political family whose work on the House Rules Committee and chairmanship of the House Democratic Caucus made him one of the best-liked and most respected members of Congress, died Jan. 20 at his home in the Watergate apartments complex in Washington after a heart attack. He was 61.

Mr. Long first served in the House from 1963 to 1965. He was reelected in 1972 and had served continuously since 1973.

At the time of his death, he was the second-ranking member of his party on the Rules Committee.

During Ronald Reagan's first term as president, Mr. Long was chairman of the House Democratic Caucus.

During his second inaugural address, in the Capitol Rotunda yesterday, President Reagan called for a moment of silent prayer in memory of the congressman.

The esteem in which his colleagues held him was one of Mr. Long's assets. In addition, he brought to his work a seemingly uncanny mastery of House procedure, a knowledge of internal House politics, and an ability to conduct complicated negotiations.

Thus, he helped make the Rules Committee -- once a major roadblock to liberal legislation -- a loyal working arm of the Democratic leadership. Although its chairman is the feisty Claude Pepper (D-Fla.), Mr. Long was regarded by many as the leading force of the Democratic leadership on that committee.

As chairman of the Democratic Caucus for four years before stepping down last month, he played a role in formulating a program for his party's future. Under his stewardship, the caucus produced a series of papers on issues. Among them was the 1982 book entitled "Rebuilding the Road to Opportunity."

In his role as caucus chairman, Mr. Long also called on his party in 1982 to "recapture the vital center of American politics." He testified before the Hunt Commission in 1981 about the need for automatic delegate representation by the congressional wing of the party at national conventions.

Another longtime interest of Mr. Long's was the economy. He had served on the Congressional Joint Economic Committee since 1973.

The committee writes no legislation and assignments to it are not sought with the same fervor as seats on some other committees. Mr. Long saw it not as a source of power in the usual sense, but as a vehicle for studying such broad questions as Japanese-American trade, world energy supplies, and the desirability of new international trade organizations.

Over the years, he gained a reputation as the quintessential moderate Democrat. He opposed President Reagan's early budget proposals, which gained great popularity with other southern Democrats, the so-called "Boll Weevils." But he refused to use his power as caucus chairman to attack party members who disagreed with him.

"When I ran for caucus chairman, I pledged to try to unify -- and not purify -- the Democratic members of the House," he said.

House Speaker Thomas P. (Tip) O'Neill (D-Mass.) said yesterday that Mr. Long had been a "close friend and strong colleague" who helped reinvigorate the Democratic Caucus.

House Majority Leader Jim Wright (D-Tex.) called him "a legislator's legislator" and praised the congressman's "constructive and creative" approach to the problems of the country. "He never sought confrontation for the sake of confrontation," Wright said.

Gillis W. Long was born on May 4, 1923, in Winfield, La. He served in the Army in Europe during World War II, winning the Bronze Star and Purple Heart medals. He was a 1949 graduate of Louisiana State University and earned a law degree there in 1951. He was counsel to several committees on Capitol Hill before his own election to Congress.

He was a distant cousin of the legendary Huey (Kingfish) Long, who was governor and then U.S. senator before his assassination in 1935. Another cousin was Sen. Russell Long (D-La.).

Despite such family connections, Mr. Long's political road was not without its rocky spots.

In addition to finishing third in gubernatorial primaries in 1963 and 1971, he was defeated for reelection to the House in 1964 by yet another relation, state Sen. Speedy O. Long, a conservative.

It was pointed out during the 1964 campaign that Gillis Long had supported the Democratic leadership on the question of enlarging the Rules Committee, a move that eventually unleashed a backlog of civil rights legislation.

From 1965 to 1966, Mr. Long served in the Johnson administration as assistant director for congressional relations of the Office of Economic Opportunity.

Survivors include his wife, Cathy, of Washington and Alexandria, La.; two children, George, of New Orleans, and Janis, a law student at Tulane University, and a brother, Floyd H., of New Orleans.