Gellman holds a B.A. from the University of Wisconsin and a Ph.D. in Philosophy from Northwestern University.  He was ordained by the Hebrew Union College-Jewish Institute of Religion and is the senior rabbi of Temple Beth Torah in Melville, New York. Gellman is a past president of the New York Board of Rabbis.

Gellman was a contributing editor of Moment Magazine where his collection of modern interpretations of Bible stories for children first appeared. "Does God Have a Big Toe?" was selected by The New York Times, Time magazine, The Los Angeles Times and People Magazine, as one of the best children's books of 1989. His second volume of modern midrashim for children, "God's Mailbox" was published in 1996. In 1997 he published "Always Wear Clean Underwear and Other Ways Parents Say They Love You." 

With his friend Monsignor Thomas Hartman, he published "Where Does God Live?" which received a Christopher Award; "How Do You Spell God?" (with a forward by the Dalai Lama) which was made into an HBO animated special and received a Peabody Award. They also coauthored "Lost and Found: A Kid's Book of Living Through Loss; Bad Stuff in the News"; and Religion for Dummies.  Gellman and Hartman host a cable television program called "The God Squad" and write a nationally syndicated spiritual advice column in newspapers across the country. They also appear on many news programs and are regulars on "Imus in the Morning."

Gellman and his wife, Betty, raise guide dogs for the blind. They have two married children: Mara, married to Ilan are parents of Ezekiel, and Max, who is married to Phoebe.