Besides Marcel Proust and André Gide, are there any other important writers of 1900-1950 whose work was considered an experiment?
Your question is too broad and too vague to address specifically. “Important” is a difficult quality to describe, and virtually all the “important” literature of the first half of the 20th century was experimental in style and/or content, since this modernistic period in world literature threw out all the “rules” of Victorian literature. Chief among the “most important” writers, if you restrict your inquiry to novelists, the narrative styles and characters delineations of James Joyce, William Faulkner and Ernest Hemingway stand out. Among poets, Ezra Pound, E.E. Cummings, and T.S. Eliot come to mind. Regarding “philosophical” writers, you may add Albert Camus and Jean Paul Sartre to Andre Gide. Drama is another area where much experimentation took place -- Eugene O'Neill for example.