ACTC has a pretty large database of Bay Area climbs (aka 'Billy Goats')
http://www.actc.org/billygoats/bgoats.php
The climb up Old La Honda from Portola Rd to Skyline Blvd is given a difficulty rating of 5 (ascends 1475 ft in 3.9 miles), while the Canada Loop, which includes the climb up to Gilroy Hot Springs, is given a difficulty rating of 4 (ascends 1020 ft in 17 miles).
It's all rather subjective, but hopefully encouraging.
[Ed to add: Take those ratings with a grain of salt. Sierra Road is also given a 5 in this database, which makes it in theory only as tough as OLH. Not !!!]