The actor sold the property in a 'handshake deal' to Kent and Pam Williams, who founded jewelry and silver company Montana Silversmiths.
The Williams made a trip to California to visit Gibson and then made a verbal deal to buy the ranch, The Missoulian reported at the time.
"Everything was on a handshake," Kent Williams said.
The ranch sits five miles southwest of Columbus, a town of about 2,000 people.
It's about a 45-minute drive from Montana's largest city, Billings, and a two-hour drive from Bozeman.
The operating ranch can accommodate about 1,000 cattle and horses.
The current owners lease out various components of the ranch, according to the listing.