Gilfillan was born and raised in Mt. Gilead, OH, where his outdoorsman father (Merrill C. Gilfillan) worked as a naturalist for the state's Department of Natural Resources and helped inspire an early fascination with the natural world and its creatures. Gilfillan graduated in 1967 from the University of Michigan. He attended the University of Iowa Writer's Workshop for two years, studying with Ted Berrigan, Anselm Hollo, and George Starbuck, among others. He lived and worked in New York City for eight years and then moved to Colorado, which serves as a base for frequent expeditions to the Great Plains and other regions of America from which he reports in essays, poetry, and short stories.