I don't think the San Gabriel typically is big enough for a good float that far upstream unless it is flooded from rains which I would consider dangerous. You might have better luck launching at the San Gabriel park in Georgetown and floating to the low water crossing at highway 29 east of town.
I agree. I just took the trip from Georgetown to the low water crossing and did more walking than paddling. Went through a couple nice chutes though where my inexperienced friend sunk his kayak twice. Actually, it was one of my kayaks and the damage he did to the bottom was worth the laughter at his antics. At one point he ran into a tree and flipped completely upside down.