congress

From Latin congressus, the past participle of congredior, "to go or come together," built from con- ("together") and gradior ("to step, to go").

The English noun came first, then gave rise to a verb form, keeping the sense of parties stepping together into one place.

It shares its "coming together" logic with words like convent and convention, which trace back to con- + veniō rather than gradior — the same idea of assembling, taken by a different verb.