The trailhead is just above Red Box picnic area on the Mount Disappointment access road. The trail heads up about a mile and ends just north of Mount Disappointment.
Just before the peak, you'll rejoin the access road and so you can easily bag both San Gabriel Peak via the San Gabriel Peak Trail
and Mount Disappointment via the access road. There was snow and ice on the upper trail in early February.
You are rewarded with a spectacular view of Mount Baldy from the top. You are also looking down at the Mount Wilson