It is across the street from the entrance of the GC. It is behind the AMC theaters in Downtown Disney. It is somewhat inconvenient to go to your car IMO. The valet is more convenient, but they charge a fortune for it! Sorry, I don't know where a map is, but if you get one, directly behind the AMC is where the lot is.
Locate the Grand Californian on this view. The self parking is the square lot about an inch to the left of the hotel. http://www.wdwinfo.com/photopost/data/556/11450dlmap.jpg
Of course when you get there it seems much farther.
That's a great map! You can see the self parking across the road from the GC. On the map it is right next to the PP. The other buildings are DD buildings, including the AMC theater.
Here is a view of the Grand Californian parking lot with the Grand Californian in the background. It's probably a 3-5 minute walk from the entrance of the hotel to the parking lot.
You won't have to lug your bags from the parking lot to the hotel. You can drive up to the hotel, unload your bags and family, and then go park the car. When you leave, you can go get your car while your family waits with the bags in the front of the hotel. Have fun!
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