The House Armed Services Commitee has voted to delete from the Pentagon's budget the money to buy a German gun for the Army's new tank.

The action is a major blow to President Carter's effort to improve relations with NATO partners by demonstrating that the United States is sincere about wanting to buy weapons from them and not merely sell to them.

The Pentagon had decided to spend millions of dollars on the German 120 mm. smooth bore gun for many of the XM-1 tanks. The decision came after years of arguing over the relative merits of it, the American 105mm. and the British 120mm. rifle-bore gun.

Upon learning yesterday that the House Armed Service Committee, in a closed markup session late last week, had eliminated the $8.1 million for the German gun, Defese Secretary Harold Brown began a telephone lobbying effort in hopes of reversing the action.

The committee is scheduled to meet today to take final action on that and other changes, in the Fiscal 1979 defense authorization bill for aircraft, missiles, tanks and other weaponry.

Brown, sources said, stressed during his telephone calls to congressional leaders that the House committee action, if not reversed, would undermine the "summit" meeting of NATO heads of state scheduled for ate this month in Washington.

European NATO nations have been complaining for years that the United States talks of a "common market" of weaponry for the alliance but fails to back it up by buying weapons from its partners.

Carter and Brown have been trying to assure European leaders since taking office that they consider NATO procurement a "two-way street."

If Brown and other Carter administration officials fail to get the money restored at today's meeting of the House Armed Services Committee, the effort will be expanded to the House floor and Senate, according to Pentagon sources.

The reason the committee voted to delete the money, congressional osurces said, was that its investigations subcommittee, headed by Rep. Samuel S. Stration (DN.Y.), has not finished its inquiry into the matter.

The $8.1 million was eliminated "without prejudice" and could be restored later in the session, according to committee sources. But it was considered doubtful last night that the money would be restored at today's meeting of the full committee, although the administration lobbying might turn the tide.

Even a delay in approving the money, however, would be a damaging blow to the Carter administration's effort to solidify the NATO alliance, defense officials said.

The whole tank controversy has been especially prickly politically. First the United States and West Germany were to build the new main battle tank together; then that plan was abandoned as both countries built their own, with the United States promising to take a hard look at the German Leopard before making a choice; then the Leopard was rejected in favor of a Chrysler tank, with the promise of buying the German gun for many of them.

The House Armed Services Committee's initial vote to pass over the gun request was one of several fundamental changes by that panel as it reviewed Carter's defense budget.

The committee also refused to go along with the administration's effort to spend $41.2 million to convert two jumbo jets, such as the Boeing 747 or McDonnell Douglas DC10, to carry between 60 and 80 cruise missiles.

The House committee argued that the transportswould be too vulnerable to attack to justify such an investment.