The close friendship between William Gass and fellow novelist William Gaddis began in the early 1980s and lasted until Gaddis's death in 1998. Gass was an early champion of Gaddis's work. In 1976 Gass was on the panel that gave the National Book Award to Gaddis for his second novel, J.R. Much later, Gass also wrote an introduction to the Penguin Classics reissue of Gaddis's first novel, The Recognitions. The "Memories of Master Gaddis" address below was adapted from that introduction.

Gass traveled with Gaddis and other writers to the U.S.S.R. in 1985 as part of a writer exchange with the U.S. Gaddis was notoriously reclusive when it came to speaking to the press or giving readings, but upon Gass's invitation he participated in symposiums and events at Washington University in St. Louis, mainly in association with the International Writers Center there. The two letters from Gaddis to Gass and the audio recording of Gass introducing Gaddis, located below, speak to their affinity to each other and their work.

It was Gass's friendship with Gaddis and Gass's championing of the Modern Literature Collection that brought Gaddis's entire literary archive and personal library to the Washington University Special Collections. The William Gaddis papers consist largely of his own manuscript material: manuscripts and source material toward his books, drafts of various stories, published and unpublished, as well as essays, reviews, interviews, and a miscellaneous assortment of notes and other materials. Also present is a substantial amount of personal and general correspondence, primarily with family, friends, and fans. In addition, there is a relatively large amount of correspondence to editors, translators, and publishers, as well as correspondence with his colleagues in the literary community.