THERE is a nationwide gender gap in philanthropy. Foundations and nonprofit organizations dole out millions of dollars less to women's organizations than to men's, according to a new study.The study, done by the Feminist Majority Foundation, was announced yesterday simultaneously with news of a $10 million endowment to help close the gap. The money was given by Peg Yorkin, who chairs the Feminist Majority board, to the Feminist Majority Fund and Foundation. It is the largest donation to a woman's organization on record. Ms. Yorkin, a Hollywood producer, said in an interview before the press conference: "I think that most people are not aware there is such a gender gap in giving. If, for instance, you look at United Way grants, men receive $105.6 million but women receive $66 million. The Boy Scouts get $87 million, the Girl Scouts get $56 million." She also points out that of grants made by US foundations in 1990, only 5 percent went to programs specifically for women. Eleanor Smeal, president of the Feminist Majority, said during the announcement: "Across the board, giving to nonprofit organizations is sex-biased. That's why we're urging all supporters of women's rights to earmark their contributions to programs and agencies that promote women's equality." Ms. Yorkin's $10 million gift is to be used to form the Feminist Empowerment Center. She says, "[It] is not a center in terms of bricks and mortar, it's a center as far as projects are concerned. We [the Feminist Majority] have three offices now - in Arlington, Va., for the Washington area, one in Los Angeles, and another in Boston - and the Empowerment Center will be run out of those."