Has the original text of the Gazeira of Rabbeinu Gershom been lost. If so when? Who are the earliest sources stating such?
The late professor Louis Finkelstein writes extensively on this topic in his work Jewish self-government in the middle ages (pdf), see especially starting on page 111.
His answer is that it is unclear if all the takkanot which are attributed to him originated with R' Gershom to begin with and that the original text has certainly been lost. He attempts to reconstruct the language from various sources he lists ad loc.
(וכן העיד הרא"ש:"ותקנותיו וגזרותיו קבועות ותקועות כאילו נתנו מסיני" (שו"ת הרא"ש כלל מ"ג סימן ח'
The answer here in the Footnotes of Halachically Speaking, Volume 5 Issue 17, "Reading Another Person's Letter"