Be careful on the way "into" the garage, because I assume after a good drive, your tires will be pretty hot, and they will really pick-up a lot of stones then.
I live on a gravel road (1/2 mile off the main road), my driveway is 200+ feet of gravel, and my garage is in back of my house, and I have to drive through a horse paddock to reach it.
I just drive slowly. I've been doing this for over 2 years now, and haven't had any problems. Although I do often joke that my Box is probably the only one that goes "off roading" whenever it gets driven.
I also live at the end of a gravel road. I have a triple black '04 S. Bought it new in Feb. '05, and drive it 3-5 times/week. I have had no problems with stones, but a never-ending battle with dust. When I wash it, it has a light coating of dust by the time I get to the end of the street.
pdx,
Yes, I hear you about the dust on black. But even with a slight coat of dust, I still prefer it to most other colors. Btw, the coat gets blown away while driving anyway. Boxsters are great for shaking the dust off.