He is most famous for the synod he called around 1000 CE, in which he instituted various laws and bans, including prohibiting polygamy, requiring the consent of both parties to a divorce, modifying the rules concerning those who became apostates under compulsion, and prohibiting the opening of correspondence addressed to someone else.

https://en.wikipedia.org/wiki/Gershom_ben_Judah

You can google Rabeinu Gershom for more detailed information.

which, around the year 1000 seemed like forever. Thus, it should have "expired" around the year 2000, but I am not aware of anyone restoring the custom or reintroducing the ban.

But permitted. (There is a distinction.)

If one looks to the Tanukah, many of the themes of the stories and events where various of the founders were to the effect that polygamy is not a good idea and had plenty of complications.

For the most obvious example, see generally: Abraham and the hand maiden. And David. And Shlomo.

There are other examples (and even requirements) when a woman is taken as a "wife" (e.g., of the former husband's brother) because she is a widow, and thus had horrid prospects back in the Bronze Age, but it's more of a formality to provide for her and her children. In these situations, things typically went much better.

If one looks to more recent times, a very encompassing mitzvah is that Jews are required to follow the laws (and also customs, to an extent) of the land in which they are in. Therefore, in the West, where polygamy was forbidden, it was forbidden for Jews. In contrast, Mizrahim in Arab lands much more recently would occasionally have more than one wife, but often for economic reasons.

http://www.chabad.org/library/article_cdo/aid/558598/jewish/Does-Jewish-law-forbid-polygamy.htm