Gambrel is sometimes what we called dutch barn style growing up, it's where the roof has two pitches and the walls are lower (ignore the dimensions, it's just a pic I found to show the style):
Gable is your traditional center peak triangle-end style where the side walls are full height.
The first style with 6' sidewalls would look pretty good. The second style, I'd want to go taller, at least 7 so you could hang stuff like ladders from the rafters without bumping into them all the time, but on a 10' wide shed 8' high walls would not look out of proportion, once built it'll look small in no time.
I know you said you just want to keep stuff in there for the winter and that during the summer it's in the garage, but switching it back and forth twice a year will get old, and you'll find yourself leaving it one place or the other all the time. Might as well make the space in the shed one that will let you move around and work on the equipment some and not just barely big enough to cram it all in there and climb over it all to get out.
Also, there will always be "just one more thing" you want to keep in out of the weather. If you take stuff out of the shed in the spring to put it in the garage to use it, OTHER stuff will "suddenly" move in over the summer. Around my place, it's usually not even my stuff!