Gary grew up in Irvine, California and later attended Newport Harbor High School and did acting training at South Coast Repertory Young Conservatory Players. As the 1980s came to a close, Gary spent much of his time following the Grateful Dead (and later Phil Lesh and Friends). Gary says he lived off residual checks for almost twenty years after leaving Hollywood. He has lived in Portland and Eugene Oregon and currently resides in the Los Angeles area.

Riley played Charlie Hogan, one of his best-known roles, in the 1986 film Stand by Me, and also received notice for his portrayal of Dave Frazier in the 1987 comedy Summer School. He made notable appearances as a thief who steals money from Steve Martin's character's wallet in Planes, Trains, and Automobiles and as a naive metalhead customer whom Judge Reinhold can not bear to rip off after discovering his girlfriend is pregnant in Ruthless People. He also made guest appearances on such TV shows as Knots Landing, Growing Pains, Amazing Stories (in a special hour-long episode directed by Steven Spielberg), and Airwolf. Riley was raised in Irvine, CA. Gary's last role to date was as a thug in Fear, alongside Mark Wahlberg. Gary participated in the Summer School Q&A at New Beverly Cinema in the summer of 2013.