The Gabrielino is my backyard, I run about 15 miles of it once a week.
Nick's info is good.
As for scenery, I love the area (of course I do, it's home). Don't compare it to anything, enjoy it for what it is. The San Gabriels are the San Gabriels...Oak trees and mellow canyons, conifers and snow in the higher country.
The Sierra is the Sierra...
I think the Angeles National Forest is (thankfully) very under appreciated. The crowds completely disappear once you're 2 miles in. Most of Los Angeles has no idea you can go snow camping in less than 40 minutes driving and hit trails that will take you 50+ miles in only minutes from Pasadena.
Go for it- if it's what you're looking for, the Gabrielino trail should be easy weather compared to many other places right now.
Pick up the Tom Harrison maps:
Angeles Front Country (I think about all of the Gabrielino should be on this one)
and Angeles High Country.