Gerald M. Cahill Sr., 77, a retired associate general counsel of the Federal Communications Commission, died of cardiac arrest Thursday at Suburban Hospital. He lived in Chevy Chase.

Mr. Cahill joined the FCC in 1958. He held a legislative liaison position, in which he worked with committees and members of Congress and helped prepare legislation for congressional consideration. He retired in 1968.

Mr. Cahill was born in Boston. He was a graduate of Georgetown University and of its law school. He was an attorney with the Home Owners Loan Corporation during the 1930s and early 1940s, resigning in 1945 to become counsel here for the Motion Picture Association.

He remained with the association until joining the FCC in 1958. At the time of his retirement, he received the FCC's Meritorious Service Award.

Mr. Cahill had been a parishioner of the Catholic Shrine of the Most Blessed Sacrament for more than 40 years. He belonged to the Kenwood Country Club.

He is survived by his wife, the former Julia Gresko, and a son, Gerard M. Jr., both of Chevy Chase; another son, John T., of Rockville, and six grandchildren.

The family suggests that expressions of sympathy be in the form of contributions to the organ fund at Blessed Sacrament.