Has anyone patented drawing a circle with one finger? Clockwise and counter-clockwise could mean different things. Go clockwise to zoom, counter to shrink. Make a larger circle once, or make several small circles. Maybe not even a circle, but start drawing a right angle, left then up to zoom, down right to shrink.
Better yet, build a generic utility to allow users to define which movements do what. Then if the user wants to pinch-to-zoom it's their prerogative, as long as the software does not mandate it. And use such utility to define gestures for anything else, like bookmarking.
Another idea, small circle with one finger extending to line, similar to digit 6, line determining how much to zoom. perhaps reverse motion to shrink.
