"Doonesbury" cartoonist G.B. Trudeau has created a boomlet of concern about the first ladies' inaugural gowns at the National Museum of American History. The first 50 pieces of mail, containing checks, pennies and a few negative comments, arrived at the museum yesterday. More mail is undoubtedly on the way in response to last week's series of strips lamenting the fact that Nancy Reagan's inaugural gown is sagging under its own weight and needs costly repairs. The money, of course, is not available because of the heartless Gramm-Rudman-Hollings act.

On Saturday, the strip called on the kids of America to send in money and listed the National Museum of American History and the address. Renee Kortum, the museum's director of external affairs, said Trudeau had contacted her and also spoke with the museum's director, Roger Kennedy, after reading in The Washington Post about the problems with the first ladies' gowns. Trudeau said he wanted to help. The museum is looking at expenses nearing $70,000 to repair six gowns. Plans are also under way for a major expansion of the exhibit on the first ladies for the next presidential inauguration in 1989.

Kortum said yesterday's mail included $25 from a Beverly Hills businessman who wrote, "Do something before it's too late"; $5.42 from an office collection at Apple Business Printers in Orange, Calif.; $5 donations from New Orleans and Anchorage; an IOU for 25 cents; a penny; a check for 5 cents from a Wilshire Boulevard lawyer; and a note from a man in Grand Rapids, Mich., advising the museum to sell the gown and give the money "to the people who lost money from Texas oil and big mouth Bush." Who knows what tomorrow will bring for one of the pop causes of the 1980s? Trudeau, as usual, would not comment on his strip. No Response to Elliott Abrams

Elliott Abrams, assistant secretary of state for inter-American affairs, told Harvard University officials that he would not share a platform last night with Robert White, former ambassador to El Salvador and a critic of President Reagan's policy on Central America. Harvard withdrew White's invitation. White had been scheduled to respond to an Abrams speech on "Democracy in Central America" at the John F. Kennedy School of Government.

Greg Lagana, Abrams' press secretary, said the assistant secretary refused to appear "because it would be a waste of time. White's views have become discredited . . . and he's become something of a crank. Secretary Abrams doesn't object to sharing a platform with critics and even suggested to Harvard officials which members of the faculty could serve in that capacity."

White said he "wasn't upset about Elliott Abrams. It's sort of predictable. What I regret is that Harvard permits itself to be dictated to." A Harvard spokesman said speakers have the right to review names of prospective respondents and that the mistake in this situation was Harvard's. End Notes

International bestselling author Theodore White ("The Making of the President . . .") is in critical condition in Lenox Hill Hospital in New York after suffering a stroke. The 71-year-old former Time magazine correspondent was brought to the hospital on Friday. A reporter for 50 years, White changed political reporting with "The Making of the President, 1960," a detailed account of the John Kennedy-Richard Nixon election battle that sold 4.2 million copies and was on the best-seller list for nearly a year. He then wrote three other "Making of the President" books -- 1964, 1968 and 1972 . . .

Ethel Shields Garrett, the Washington civic leader who died March 15, not long before her 90th birthday, worked for more than 25 years to rescue the original sandstone columns removed from the Capitol's East Front when it was expanded in 1959. Yesterday, Vice President George Bush presided over ceremonies honoring Garrett and marking the beginning of the columns' installation on the National Arboretum's grassy knoll . . .

Jihan Sadat, the widow of Egyptian president Anwar Sadat, was the luncheon speaker yesterday at the National Press Club and was asked if she ever expected to see a woman leader in one of the Arab countries during her lifetime. She said: "Only if it's Utopia" . . .

Harry Teter Jr., general manager of the National Theatre, was standing in front of the theater Monday night before the premiere showing of "Samaritan: The Mitch Snyder Story" when he observed that it marked the first time the theater had been used for a feature film in about 35 years.