Sounds like a great ride. Not sure of the average gradient but imagine it's no more than 5%. Start early as there is little cover from the sun. The climb is pretty consistent and traffic is low on Big Tujunga. Good water stop/rest area at the Clear Creek information center located at the Angeles Forest/Angelest Crest "T".
If you're planning to attack upper Big T too, there's a small store to fuel up after the tunnel