Two things about these ordinances are less settled than their status suggests.
The first is authorship. Gershom ben Judah of Mainz, called the Light of the Exile, died in about 1028, and his standing was such that later enactments of the Rhineland academies were attributed to him as a matter of course. Historians working from the manuscript record, Louis Finkelstein among the earliest, have argued that several of the ordinances that travel under his name were enacted by synods after his death and gathered under his authority afterwards. The core is generally accepted as his; the edges are not.
The second is duration. A tradition reported in later authorities holds that the ordinances were enacted only until the end of the fifth millennium, that is until the year 1240 of the common era. Nobody acted on that expiry. The rules were reaffirmed, treated as accepted custom, and in the case of the monogamy rule reinforced by the sanction of a ban against anyone who breached it, so the question of formal expiry never became practical.
Their geographical reach was always limited. The ordinances bound Ashkenaz, and Sephardi and Mizrahi authorities did not adopt the monogamy rule on the same footing, though most communities restricted polygyny by other means such as a clause in the marriage contract. The practical divergence was closed in Israel in 1950, when the Chief Rabbinate applied a monogamy rule to all communities under its authority.