I don't have specific advise on a GPS system.
Your SFO to LA options are many, depending on how much time you have & what you want to see.
The Interstate-5 route is most direct, but boring.

A more interesting route would include Monterrey-Carmel area and Big Sur, on Hwy 1 along the coast, then cut it to 101 to hit Santa Barbara on the way to LA. (You can plot these routes on Google maps.)

For sightseeing ideas, check out the California tourism board website. This page has an whole-state plan from south to north that takes 5 days. For SFO to LA, see days 2-3-4 in reverse order.

http://www.visitcalifornia.com/Details/e55101/

And remember to drive on the right hand side!