When Soviet President Mikhail Gorbachev visits the U.N. General Assembly this week, it will be the first time a top Soviet leader has appeared there since 1960.

The visit also will present President Reagan and President-elect Bush with a unique set of problems and opportunities.From the U.S. point of view, this meeting - which can best be described as a non-summit summit - could have benefited from more planning. Acting through his Ambassador Yuri Dubinin, Gorbachev sprang his intention to visit the U.N. with little notice and no consultation. But that didn't keep Gorbachev from inviting Reagan and Bush to meet him in New York so he could say goodbye to one and get acquainted with the other.

Their seeming to go hat in hand to see the important visitor between his chats with diplomats and side-trips to Wall St. can put the White House on the defensive and lend undeserved weight to whatever proposals Gorbachev may make in his address at the U.N. The Soviet leader has a habit of propounding sweeping arms control plans in a way that makes old schemes with dangerous pitfalls look deceptively new and attractive.

On balance, however, the U.S. has more to gain than to lose from the Dec. 7 get-together in New York.

At the very least, meeting with Gorbachev can help smooth the transition between the Reagan and Bush administrations. It also enables Bush and Gorbachev to size up each other's strengths and weaknesses without all the political risks and exaggerated expectations that can accompany a formal summit.

For Reagan, the meeting provides a chance to wrap up his presidency on an active note. For Bush, it is a chance to emphasize continuity in U.S. foreign policy and get a head start on some of his new responsibilities six weeks before entering the Oval Office.

If U.S.-Soviet strains develop early in the new administration, Bush will be able to use the New York meeting as a reason for putting off a full-fledged, working summit with Gorbachev.

But if the New York meeting goes well, Bush will be enabled to review U.S.-Soviet issues with Gorbachev at a moment when the President-elect's formal responsibilities are minimal.

Although President Reagan has made great progress in reducing world tensions, foreign policy issues must remain high on Bush's agenda - and Russia is at the center of them.

Though the Cold War seems to be abating, it's far from over yet. While the communist system is in trouble in Russia and crumbling around the edges in Eastern Europe, the Kremlin is still ambitious and can be aggressive. If Gorbachev fails in his reform efforts or is ousted, George Bush could find himself presiding over the escalation of the Cold War rather than its reduction. Or, if Gorbachev succeeds, a more modern and sophisticated Russia could also be dangerous to the world in more subtle ways.

Already, deep differences have arisen within NATO over how to respond to Gorbachev's sweeping reforms, over trade issues, and over the future role of conventional arms in the defense of Europe.

In short, Bush can now get a running start in dealing with some of his most important new responsibilities. Bush and Reagan clearly did the right thing in accepting the opportunity presented by the Gorbachev visit despite the problems it involves.