I’ll assume that the garage ceiling is mounted to the floor joists of the bedroom and that you have access. Teh best way is to remove the ceiling and apply insulating batts to the floor above, is not too practical. The easiest way to do this is to blow-in insulation. Either cellulose or fiberglass is fine. Be sure to fill the end-walls where the joists meet the walls. That ceiling is a fire-rated drywall, and its important that you restore the integrity of that fire-barrier when you are done. You should be able to patch in fire-rated drywall plugs where you cut ports to blow in insulation.
In Northern CA, insulation is important not only for cold protection but to keep heat from rising to the living space in summer. Blown in insulation will work fine for that purpose as well.
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