Anyone can
enter the hotel from either Downtown Disney or DCA at any time. You can then
exit the hotel to Downtown Disney any time (thru Security). You can
exit the hotel into DCA (thru Security) during the day as long as you have a ticket
EXCEPT only guests can go from the hotel (thru Security) into DCA for "30 Minute Early Entry" before the park opens on Mon/Wed/Fri/Sun I think (be sure to check), as that's for hotel guests with tickets and room reservations only - they do check both.
But to clarify once more - nobody can enter the Grand Californian from the
street (Disney Way) on foot or with a car unless they have a confirmed hotel or hotel dining reservation. There is a locked gate for pedestrians who have already checked in, and a manned guard shack to stop cars. They WILL turn you away. Anyone can leave lol, but not enter (gate can be opened from the inside by anyone leaving on foot). There are security cameras, and I've seen security appear more than once when someone tried to come in without a room key