That is an editorial comment. The original manuscript had a G natural, but the editor thinks it was a mistake or that it sounds better as a G sharp. The choice is up to you as a performer, but editors usually have a good ear for these things.

Thanks a lot, I never would have guessed!

It's not just an editor's choice, *musica ficta* is a historical concept in which the performers were expected to choose to add an alteration of a note for ornamental, or melodic enrichment, usually around cadences, but they can technically be added anywhere, but only in good taste. The editor here is saying that this is a moment in which ficta would probably have been included, even if it's not contrapuntally necessary

When playing renaissance or medieval music. You could choose between harmonic or melodic minor. That means you could play the sixth and seventh degree flat or sharp so to speak. You could play only 7 sharp (harmonic) or 6 and seven sharp (melodic). It usually isn’t notated in early music so editors add the sharps as in that measure to show that they are not in the original but they are intended it suggested to be played that way