A federal grand jury in Newark, N.J., is investigating former Rep. Edward Garmatz (D-Md.) who served in Congress 25 years and was chairman of the House Merchant Marine and Fisheries Committee when he retired in January, 1973.

According to documents filed last week with the House clerk, the grand jury subpoenaed Garmatz' congressional campaign fund records for 1970 and 1972, and his House expense reports from Sept. 9, 1971, to Jan. 3, 1973.

The subpoena also called upon the House clerk to produce 1971-1973 expense accounts of Robert McElroy, who is those years was chief clerk of the committee Garmatz chaired. McElroy died in May, 1974.

Since he retired, Garmatz has been employed by the International Organization of Masters, Mates, and Pilots, one of the strongest of the maritime unions. According to an associate, Garmatz is a consultant to the union's trade school, the Maritime Institute of Technical and Graduate Studies, located in Baltimore.

Garmatz was unavailable for comment yesterday. Assistant U.S. Attorney Frank C. Razzano, who is handling the Garmatz investigation, yesterday refused to comment on the case.

According the news reports, Garmatz collected $37,000 for his re-election campaign in 1970 from maritime unions and shipping industry sources even though he had no opposition that year.

In October, 1971, a fund-raiser was held for Garmatz by industry donors. Some of the $15,000 reportedly cleared by that function was kept by Garmatz after he announced in early 1972 his intention to retire from Congress at the end of that term.