What else might work to zoom?
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.
Two-finger drag up to zoom-in, centered on where you first start the drag. Two-finger drag down to zoom-out, centered on where you first start the drag.
Draw a circle (user can set whether clockwise or counter-clockwise) to literally outline what you want to zoom-in on. Draw a circle in the opposite direction to zoom-out, centered on the centroid of the circle.
Similarly to the reverse function in Paper by 53, maybe a two-finger circular motion to zoom-in and zoom-out. Alternatively, this could be a neat way to navigate forwards and backwards through your internet browser history.