This question came up in a discussion on another question about Jedi Force Ghosts.
Was Obi-Wan the first Jedi to transform bodily into a Force Ghost?
On various other questions, people have made various (and contradictory) claims about Jedi knowledge and Force Ghosts.
Some have said the Jedi knew nothing about Force Ghosts and therefore individuals don't retain their personality after death, and that even the Sith doubted the ability to become one with the Force after death. Accepting this answer means that Obi-Wan was the first to transform his body into the Force.
Another question has an answer claiming Darth Vader learned the technique from Darth Sidious. Which would mean others have transformed bodily into the Force before Obi-Wan. (Nota Bene: The source novel for that claim, Dark Lord: The Rise of Darth Vader, is not on the list of canon Star Wars works.)
And then there is an answer here that claims both Jedi and Sith knew of becoming Force Ghosts after death, but that the Sith rejected this method of immortality. This would imply Obi-Wan was not the first to convert his body into a Force Ghost. The knowledge for that claim comes from the novel, Darth Plagueis, which is non-canon. If that novel was declared canon, it would contradict other parts of G-canon.
Can anybody provide a real canon answer about the history of Force Ghosts?
I am looking for answers from G-canon, not lower canon levels.