John, the gap really doesn't matter so much on mopeds so long as it isn't too big. As long as it electrically breaks, it'll spark. By changing the gap, you change when the points cam follower engages the cam lobe thereby changing your ignition timing. On a vespa, when you change the gap you change the timing and vice versa.
Since point dwell isn't really an issue on mopeds (they seem to run fine at any reasonable gap, likely because they're single cylinder engines) timing is usually the driving factor when it comes to ignitions with stationary stators (like vespa). For people concerned with performance, point gap typically isn't measured, rather when the points break with respect to cranshaft position before top dead (or, ignition timing). This is why we say "point gap doesn't matter" for vespa - because the measured gap doesn't matter. What matters is when the gap starts to happen (the points break) with respect to flywheel rotation.
Hope this answers your question. (edited)