Time Out says
This 210-adjacent rock in Pasadena is known for its craggy profile of Satan, naturally chiseled into an outcropping on the Arroyo Seco. Here are the only facts you need to know: Devil’s Gate lent its name to a nearby dam and gated channel, constructed in 1920 to control the now feeble flood waters from the San Gabriel Mountains. But fall down the rabbit hole of paranormal enthusiast websites and you’ll find a few common threads involving Aleister Crowley (infamous occultist), L. Ron Hubbard (infamous Scientologist) and Jack Parsons (infamous JPL co-founder), as well as mentions of Native American lore, “moonchild” rituals and the Seven Gates of Hell.
To reach Devil's Gate, park at end of La Canada Verdugo Road (west of Vista Laguna Terrace). Walk toward the dam path and, before crossing the spillway, turn left into the small tunnel. Follow the dirt path down the hill. Once down there, don't follow the well-worn path to the south; instead head right through a thicket of trees and branches. It's just a short walk from there to Devil's Gate. Depending on the time of year, the path may be wet or flooded.