IN A MIDSUMMIT interview, President Reagan suggested that Mikhail Gorbachev had abandoned the Soviet Union's traditional global ambitions. But you can't yet prove it in the Persian Gulf, where Mr. Gorbachev turns his back on his pledge to penalize Iran for its thwarting of peace, and least of all in Afghanistan, where he evaded Mr. Reagan's call to set a date for full expeditious withdrawal of the 120,000 Soviet occupying troops. Instead, he inserted the unacceptable condition that, as withdrawal begins, the United States must start cutting off arms and ''financial supplies'' to the resistance. Moscow's own Afghan clients would evidently continue to be eligible for arms and aid.

The two men were at pains to emphasize that the line between them remains open on the Afghan question. Their readiness to keep talking about issues that resist solution was one of the more satisfying results of the summit. Openness to discussion, however, cannot conceal Mr. Gorbachev's persisting refusal to face up to the mistake the Kremlin made -- and, more important, the crime it committed -- in invading Afghanistan. It is not simply that the Soviet army has been mauled and pinned down by guerrillas, but that the Soviet army has no right to be in Afghanistan. It is committing aggression. It has spent eight years killing and uprooting the Afghan people and destroying their land.

This is why the face-saving exit Mr. Gorbachev may well be looking for probably doesn't exist. Face-saving requires setting up a new government, or interim government, that, as he said in Washington, is neither pro-Soviet nor pro-American, that is nonaligned, neutral. On superficial reading, it may sound fine to some. But, unsurprisingly, there apparently is nothing in the Afghan spectrum that could be called pro-Soviet in the sense of an element that could survive without heavy armed guard. That is why so far all Soviet withdrawal formulas rest on a demand for military advantage to offset political weakness. Sometimes ''communists'' and ''fundamentalists'' are posed as matched extremes that might be traded off against each other. But fundamentalists have a following that equips them for a political role, and communists, discredited by their tie to a brutal foreign invader, do not.

Diego Cordovez, the United Nations' mediator, is hunting for a political way out. Maybe he will find one. It remains to be shown, however, that Mr. Gorbachev has any option that does not proceed from Mr. Reagan's promise to play a helpful role if the Soviets will but get out.