One of the nicest XC loops in the San Gabriels starts at the west Mt. Williamson trailhead, heads up to Williamson, then curves along Pleasant View Ridge to Burkhart Saddle, takes the High Desert Trail to the PCT, follows the PCT to Eagles Roost, and then finally follows the PCT east back to the west Mt. Williamson trailhead.
Here's a map. There's a good campsite (quite a few actually) on the un-named peak between Pallet Mountain and Mt. Williamson at point "G" on the map. There's no water of course, but if there's snow, then lack of running water isn't as big of an issue.
The problem with the loop is that the section of the PCT from the High Desert Trail to Eagles Roost is closed to protect some frogs (Points "M", "N", "O", and "P",), but with a relatively short car shuttle, you could come out at either Buckhorn ("T") or Cloudburst Summit ("Y"). If you do come out at Cloudburst Summit, skip the first section of the PCT after Cooper Canyon Trailcamp and just take the dirt road ("U", "V", and "W"). The PCT is stupidly routed through here. The upper section of the PCT isn't too bad ("W", "X", and "Y").
Cooper Canyon Falls is a really nice spot, by the way, although it's a scramble to get down to the base of the falls.
HJ