He was born in the Riverdale neighborhood of the Bronx, New York City, and grew up in Woodmere, Long Island.

[8] In 2017, at the Hudson Theatre, Gelber played Louis/Billy in the limited-run Broadway revival of the Stephen Sondheim/James Lapine musical Sunday in the Park With George starring Jake Gyllenhaal and Annaleigh Ashford.

[13] Gelber has a recurring role in the CBS TV show Elementary as the medical examiner Dr. Eugene Hawes.

His film credits include a role in Riding in Cars with Boys opposite Sara Gilbert, the 2007 crime drama Before the Devil Knows You're Dead and The Taking of Pelham 123 in 2009.

In November 2009, Gelber married Marsha Loeb, a senior online medical editor at McGraw-Hill.