Gladiator II is likely set about 19 years after the original, in the year 211 AD, during the reign of Emperors Caracalla and Geta. Quite a bit happened between Gladiator and its sequel, like Paul Mescal’s Lucius becoming an adult, so 20 years seems like a good amount of time to fit so many developments in.
https://screenrant.com/gladiator-2-timeline-sequel-explained/