Gilder took an active interest in all public affairs, especially those which tend towards reform and good government, and was a member of many New York clubs. He was one of the founders of the Society of American Architects, of the Authors' Club, and of the International Copyright League. He was a founder of the Anti-Spoils League and a member of the American Academy of Arts and Letters. He was a close friend of George MacDonald, Scottish poet, author, and preacher. They collaborated in various ventures such as MacDonald's American lecture tour in the 1870s. Gilder received the degree of LL.D. from Dickinson College in 1883.