I agree that the space between the house and garage both in the attic and below it needs to be walled off with 5/8 drywall, however there is no reason your garage ceiling needs to have a fire barrier from the rest of the garage.
For something different and as long as you are not storibng stuff above the garage. Place chicken wire or something similar on top of the joists in the attic space and roll out attic insulation blankets over it. Then for a ceiling use Vinyl Siding Soffit material. Choose a white color and you are all set, no need to paint ever and plenty of reflectivity for the light. Stagger the length of the pieces so you don't end up with one big seam OR use J channel to hide the edges with a uniform seam.