This page includes information about several excellent hiking trails [1] you can explore on your own, as well as organized outings [2] with fellow San Gabriels supporters and led by knowledgeable wilderness advocates.
Links:
[1] http://www.sangabrielmountains.org/places_visit#trails
[2] http://www.sangabrielmountains.org/places_visit#outings
[3] http://www.sangabrielmountains.org/places_visit/west_fork_national_bikeway
[4] http://www.sangabrielmountains.org/places_visit/east_fork_trail
[5] http://www.sangabrielmountains.org/places_visit/san_antonio_creek_falls_trail
[6] http://www.sangabrielmountains.org/places_visit/middle_fork_lytle_creek_trail