that actually already existed in the Julian calendar, put in place by Julius Caesar. the only change that Gregory made was to eliminate 3 leap days every 400 years to make the calendar year better match the solar year

also, Caesar didn’t insert July and August into the calendar, he just renamed the months, which were previously called Quintilis and Sextilis, but were still the 7th and 8th months. those already didn’t match up way back then, because they refer to an even older calendar, where the first month was March, because the year was considered to begin on the spring equinox, not the winter solstice