Most codes are on line and you can look up the requirements.
There are books of archaic materials referenced by many codes, and they spell out the presumed fire rating of archaic materials and assemblies. It is very likely that your assembly has little or no fire resistance. Fire resistance provides protection for structural members so they don't fail. Your existing condition appears to have unprotected structural members. It would not matter if you had a foot of wood on top, if the joists fail it is worth nothing.
Assemblies have fire resistance ratings, materials do not. Type X wallboard has no fire rating, but it is used in systems that have fire resistance ratings. All the details have to be followed according to the listing, including fasteners and spacing, penetrations, seams, etc. Otherwise you only have a wallboard covering without any fire resistance rating. By listing, I mean in a manual of fire resistance tested assemblies by UL, GA, Factory Mutual or equivalent accepted testing laboratory.
If you simply want to build an assembly that is fire rated for 1-hour from the garage side, look at Gypsum Association assembly RC2601/FC5406. It used two layers of 5/8" GWB type X but the instructions for support and attachment, etc, have to be followed. You will probably have to add support members.
Take a look at the assembly in this link. It is typical of the instructions for fire resistance rated assemblies. If you go to the building department and tell them you are using GA2601/FC5406, if they know what they are doing they will understand and know what to inspect. The link is an RC assembly, so it works for roofs and ceilings. It is also FC for floor/ceiling.
http://www.americangypsum.com/data/design_library_pdfs/RC2601.pdf
Or take a look at this:
http://glgsupply.yourwebhosting.com/page/resources/gypsum_pdfs/ga-610-02.pdf
There are hundreds of assemblies available, so you have to research and find the easiest to use in your instance. That is what an architect does as part of his paid service. When I was a code enforcer, I wanted an assembly number and I brought the manual with me when I inspected it. Without an assembly number, no building permit from my office. You didn't need to hire an architect, but if not you had to know what you were about to do.
Incidentally, it is not a fire wall; most codes assign a different meaning to those words. Most codes call it a fire barrier, fire partition or a ceiling with a fire resistance rating. Calling it the right thing will help the building department believe that you know your stuff.
