Gregorian calendar has 2 different meanings across 2 categories:
the solar calendar now in general use, introduced by Gregory XIII in 1582 to correct an error in the Julian calendar by suppressing 10 days, making Oct 5 be called Oct 15, and providing that only centenary years divisible by 400 should be leap years; it was adopted by Great Britain and the American colonies in 1752
The calendar currently used in the western world. It replaced the Julian calendar and was devised to halt the slow drift of the vernal equinox towards earlier in the year.