How many Pope Gregorys were there and which one invented the Gregorian chant?

So far there have been 16 Pope Gregorys in the Papacy, but only one has credit for the Gregorian chant.

The first one, Pope St. Gregory the Great, served from 590-604; the last, from 1831-1846.

It was the earliest Gregory to whom the Catholics attribute the chanting. However, he didn’t really invent it.

It’s just that the church determined centuries after the fact that at some time during the reign of Gregory I and his successor, Gregory II, a form of musical notation was created to preserve older Roman chants. Because he was pope at the time, the church decided to give him credit.

Incidentally, it was yet another Gregory, Gregory XIII, who, in 1582, commissioned the Gregorian calendar.

Not that he actually invented the calendar; he merely picked the best proposal submitted to him.