Gustave Fayet once owned enough Gauguins to turn his Paris home into a de facto museum dedicated to the artist. Fayet also organized one of the first French outings of the work of Pablo Picasso, at the time not yet 20 years old. Jeanne Lanvin commissioned Fayet to create rugs for her apartment, and Paul Poiret designed Fayet’s daughter’s wedding dress. For someone so central to the cultural scene of early-20th-century France, Fayet remains relatively unknown. The forgotten multihyphenate emerges from obscurity this fall, as the Fondation Louis Vuitton unveils Gustave Fayet, Collector–Designer , the next in its monumental Icons of Modern Art exhibition series, on view October 9 through March 8.…