For the next three months the National Gallery of Art will host a major international exhibition, "Post-Impressionism: Cross-Currents in European and American Painting, 1880-1906."

For the first time, four major French post-impressionists -- Cezanne, Gauguin, Van Gogh and Seurat -- will be exhibited alongside their contemporaries, both European and American. The exhibition, which opened yesterday and will continue through Sept. 1, contains 300 paintings, some of which have never been seen in the United States before.

The exhibit probably would not have been possible without the financial support of General Telephone and Electronics Corp. This is the first time the company has sponsored an exhibit of this scale.

"We'd been looking for an opportunity to have an event in Washington," said Alfred Viebranz, GTE senior vice president.

It is an event that almost didn't take place. A major exhibit from Leningrad's Hermitage Museum, scheduled for the spring and summer, was canceled abruptly following the Soviet invasion of Afghanistan, leaving the gallery four months to put together a show that normally would take two years to mount.

GTE stepped in and provided a substantial grant to assist in the undertaking.

Corporate sponsorshop of the arts has become big business in recent years. According to the Business Committee for the Arts Inc., American companies contributed $250 million to the arts in 1978, compared to $22 million in 1965.