It might have been any Washington black-tie dinner, except that the gossip going around was 200 years old. More than 100 politicians and historians gathered last night at the National Portrait Gallery to commemorate the Sept. 3, 1783, signing of the Treaty of Paris.

It also officially opened the Gallery's exhibition "Blessed Are the Peacemakers," portraits and busts of the people surrounding the treaty. Guests dined on pate' au cognac (served because pate' was invented the same year the treaty was signed) and John Adams Mountain Trout (because Adams was a staunch defender of fisheries).

The original treaty was the guest of honor--the British copy was flown in for the evening, and tomorrow it will appear at the White House for Prime Minister Margaret Thatcher's visit, then will be returned to London. (The American copy is in the vault at the National Archives.) Exhibition curator Beverly Cox was asked which copy of the treaty was more prestigious. "Depends on which side you're on," Cox laughed.

James E. Mooney, director of the Historical Society of Pennsylvania, said the men involved with the treaty all had their portraits painted for the signing by artists like Gilbert Stuart and Sir Joshua Reynolds. "They called Stuart's portraits 'Stuart's $100 bills,' " Mooney said, "because whenever he needed money for booze, he'd crank out another portrait."

John Adams paid 50 pre-1983 pounds for a full-length portrait done by John Singleton Copley, "but the ceilings in his home weren't high enough to hang it, so it went to Harvard," said exhibition organizer Fred Voss.

"People think we achieved our independence when it was declared," said Joan Challinor, who is a professor of early American history at American University and knows such things. "They don't realize that we achieved independence with the signing of the treaty and not before. I think it is important to know how successful peace treaties are put together . . . If we work hard enough, we can achieve peace today."

Challinor set up a national committee to commemorate the treaty's bicentennial, which has commissioned the "Fanfare for Peace" by Stephen Douglas Burton, premiered by the National Symphony, and a ballet to be performed in April by the San Francisco Ballet. It also organized a symposium on historic peacemaking and peacekeeping, which begins today at American University.

Brian Urquhart, undersecretary general for special political affairs at the United Nations (often called the "peacemaker" for the U.N.), said in his address that the treaty "is in many ways a microcosm of the fate of all peacemakers . . . Nothing is more easily forgotten than success . . . failures get all the publicity now."