The International Residential Code (IRC) and the International Building Code (IBC) require separation between private garages and the dwelling unit.
Here are the code sections that would apply:
2006 IRC, section R309.2.
2006 IBC, section 406.1.4.
In this case, since this is a detached garage, we are talking about separating the private garage from the sleeping room (bedroom).
Technically, the sleeping room does not qualify as a "dwelling unit". BUT, the codes do not permit openings from a private garage directly into a room used for sleeping purposes. So you will have to deal with that first.
I don't know why the garage ceiling needs to be sheetrocked. You do need to separate the garage and its attic from the sleeping room. Typically 1/2-inch G.B. on the garage side from floor to roof sheathing will take care of that.
Definition of "DWELLING UNIT" according to the IBC. A single unit providing complete, independent living facilities for one or more persons, including permanent provisions for living, sleeping, eating, cooking and sanitation.
Commentary: A dwelling unit, as stated, is a residential unit that contains all of the necessary facilities for independent living. This provides a single, independent unit that serves a single family or single group of individuals. This terminology is used where separation issues are required for two or more units under one roof, or for the description of the separation of a garage from a dwelling unit.