A gear is a mechanical linkage that is used to transmit rotational motion. It can be used to increase or decrease torque and speed. I started my career working for Chevrolet Muncie, and later was the gear engineer on GM's EV1 drive unit, so I have an intimate knowledge of this type of gear.
It is also used to refer to equipment used for many things. Camping gear, fishing gear, hiking gear, audio gear, scuba gear, my gear or your gear are some examples of gear (George Carlin called it stuff, but that's another tale). The point being that gear is something we use to assist us in doing other things, usually to make life easier. Our gear can be a necessity or just something that appeals to us. Usually gear affects our senses in some way, by sight, touch, smell, sound, or taste
Head can mean the top, the start, the wisest, most powerful, or just the place that our brain lives. Our brain is what controls us, gives us our intelligence. It tells us right from wrong, and gives us our emotions. It's these emotions that are at the heart of us. It gives us our passions.
In a nut shell (or cranium) a GearHead is one who has a passion for an object that they become attached to. Though I believe the term Gearhead is mostly used in the realm of cars, bikes, boats, and planes or the transportation segment. Why, because people are passionate about them, so passionate that they usually cannot leave them alone. They have to customize them, usually doing the work themselves when they can. I'm not saying that there are not GearHeads in other areas, but for the last hundred years transportation has been at the head of the class. That said, I also believe that this era may be winding down and electronics will be the passion of the next generation of GearHeads.
The other characteristic of a GearHead is that because of their hands-on approach they can be skilled craftsmen. They are the example of do it right, the best you can, and they take pride it what they have done.
Most of all GearHeads are pretty cool and good to have around.
Next time, what makes me think I qualify.