The full hike to the Great Gallery is about 7 miles round-trip and five or more hours, with a steep descent into the canyon and a climb back out. High Gallery is an earlier stop, about 1.8 miles from the trailhead. The dirt-road approaches are long and can change quickly with rain or drifting sand, so check current road information before leaving.
There is no dependable water in the canyon; bring what you need for the whole outing. The west-rim trailhead campground has a vault toilet but no water, before you start the hike. Dogs are not allowed on the trail or below the rim.