Gary Gayton, acting a administrator of the federal Urban Mass Transportation Administration, announced yesterday that UMTA will approve a $59.4 million U.S. grant so that Washington Metro can open bids as scheduled on a new order of subway cars.

The announcement, along with other actions, frees up to a total of $84 million in federal and local funds to buy about 90 new subway cars and breaks loose the most important pending action Metro has with UMTA.

The new order of rail cars must be placed almost immediately, Metro officials have said, for the cars to be available by early 1981 when Metro is planning to open an extension of the Red Line between Dupont Circle and Van Ness-University of the District of Columbia.

By that time, the Blue Line will have been extended from Stadium-Armory to Addison Road in Prince George's County, the Orange Line will have been opened from Rosslyn to Ballston in Arlington County, and Metro will need all 300 cars it now has.

Bids will be opened on the ner car order May 23 and Metro is hoping to receive offers from four manufacturers, including the only American firm left in the business, the Budd Co.

The decision to release the grant was made yesterday morning at a meeting of federal officials including representatives from UMTA, its parent Department of Transportion, the Office of Management and Budget and the White House.

Action on the grant had been delayed in recent weeks while U.S. officials sought assurances from local governments that they would pay one-third of retiring $1 billion in bonds Metro sold for construction.

That issue remains on the table along with questions about what segments of Metro's planned 101-mile system should be built next. Metro has proposed a three-year program that would see some work on five different lines by only one new segment being brought to full operation.

Metro also has proposed that it and the federal government sign at interim agreement on the $1 billion debt until the local governments have found the financing to pledge themselves to assume the one-third share.

Federal officials who attended the meeting said yesterday that a counter-offer to Metro's proposals would be sent in a few days.

"We'll put on the table our version of what an interim agreement should contain," assistant transportation secretary Mortimer Downey said. "And we'll probably have some suggestions on how some of those [segments of proposed construction] can become operable."

The general reaction of federal officials, according to a White House source, was that the Metro board in a recent letter "had made some positive suggestions."