Thanks, I didn't know that. There is no habitable space above the garage.

You still don't know it.  The requirement isn't for drywall per se, it's for a fire rated partition of a certain hour rating.  It does not have to be GWB (there are other systems that qualify), but if it is, then it's not just thickness. Type, joints, penetrations, etc.

I suggest USG Fire-Resistant Assemblies Catalog (English) - SA100 (PDF) for some light reading.

This is relevant to my interests, I've just finished renovating my office, next step is the garage.  To make life more fun, for the ceiling, there is plumbing and A/C ducting hanging down below the ceiling joists, so I might need to consider some kind of suspension ceiling.  The walls aren't really insulated very well, and I know I have leakage from the house into the garage as far as cool air getting in there.  I'll have to leave access panels for the spots on the walls where there are water valves for the incoming main water line and the two exterior faucets.  Bottom portions  of the walls are cinderblock, upper portions wood frame (and not even between the two sides of the garage).  

I used OSB and paint, couldn't justify the much higher cost of anything else

Just use dry wall it is the cheapest solution. There are a ton of guys on Craigslist that will do the job quick and cheap.

I bought galvanized polished pegboard off of amazon. That and the blue add on hook kit...did a huge wall. not cheap...but hell it looks good

There is only one guy who drywalls around here, he charges a fortune. The next guy is an hour away but he's a drunk.

The guys building my house joked that if I did a good job they would hire me for all their next jobs

In for different ideas

I am required to drywall the entirety of my garage, including the ceiling with 5/8ths.
Even though what I am building what amounts to an addition onto my existing garage and it already has a firewall between it and the house.
Reason in my case is the attic space will be continuous between the new garage, old garage and the house.

Engineer told me I could argue it and possibly sidestep the requirement by adding a firewall in the attic between the garage and the house but we agreed it wasn't a good hill to die on with the planning department.
That and I planned on insulating and drywalling the whole thing anyway.