According to the opinion quoted by Tosafos that says that there must be an explicit heter put forth for a prohibition or cheirem – even one with a specific time period – to expire, the cheirem of Rabbeinu Gershon would still be in effect since no one explicitly permitted it. This is so even though the cheirem was initially made for a specific time period.
I found afterwards that I was mechaven to the Yam Shel Shlomo in Yevamos that says that even if the cheirem of Rabbeinu Gershon was only in effect until the end of the fifth millennia, the prohibition would still apply since no one explicitly permitted it. He cites the Gemara in Beitzah as a source that even when a prohibition was made for a specific time period, it nonetheless requires an explicit heter to remove the prohibition.