The gonne appears to fire itself at times. It is not unknown for machinery on the Disc, especially new machinery, to become if not sentient at least independently active enough to make demands. So I choose to think that the gonne manifested another bullet.
The Guitar
Combine Harvester
locomotive
The problem with the Gonne is, I'm not sure if it's ever made clear that the Gonne does manifest a new bullet, or even can do it. Yes, the Gonne has its own voice; it is sentient. But what power does it have? We don't know (and we'll never know, since
Vetinari destroys it at the end of the book).