Gahagan was one of the many artists in the late 1940s that were able to pursue their studies thanks to the G.I Bill. After serving in the Navy during World War II, he attended Goddard College in Plainfield, Vermont (1947-51) where he studied with two former Hofmann students. He soon moved to New York, where he himself began attending the Hans Hofmann School of Art and became involved with the then-burgeoning abstract movement. His visual vocabulary developed during his years as Associate Director of the Hofmann School and as Hofmann's assistant during the creation of two major mosaic murals in New York City.
A staunch activist for artists’ rights, Gahagan was a motivating force among his New York contemporaries, acting as a principal organizer and the first elected President of the Artist Tenants Association. In this role, Gahagan led a successful strike by artists to win zoning for artists’ lofts in New York in 1962. This resulted in the establishment