This will be pit-run gravel on undisturbed soil, several inches thick. I will use 8" x 8" hemlock sills resting on concrete blocks. There will be no poured concrete: the garage floor will be gravel (for a few years at least). The garage size is 24' x 24'. Perimeter drainage on three sides. My location is New Brunswick, Canada so we have ground freezing in the winter. How many inches of gravel should I use? Is my plan reasonable?

Bit of an issue between too thinand too deep. Too thin and frost heave will be an issue, too deep and you will bog down unless the gravel is fairly large size which beats up the tires. I'd probably go with 4-6" of #7 stone topped with 2-3" of crusher run-sometimes called crush and run. The heavier stone may keep down heave and the crusher run will fill in the voids and give you a fairly smooth surface. Smaller country driveways are built like this in GA, where I live.