'gregarious' originates from Latin, specifically the word 'gregarius', where 'grex/greg-' meant 'flock, herd'.

'gregarious' entered English from Late Latin 'gregarius' (via scholarly/learned borrowing) and developed into the adjective 'gregarious'; the adverb 'gregariously' was formed from that adjective.

Initially it meant 'pertaining to a flock or herd'; over time it broadened to mean 'fond of company, sociable' when applied to people.