In many versions of the Talmud there are textual emendations made by "Gilyon". Who is this Gilyon and where did the publishers of the Talmud get this material from? The Gilyon I'm referring to is quoted in the hagahos ve'tziyunim section of the Oz Vehadar version of the Talmud and the different girsaos that are offered are many times sourced simply as "gilyon". (I'm not referring to the gilyon hashas by Rabbi Akiva Eiger.)
Gilyon is Hebrew for margin. It's referring to the marginal notes found in the Vilna Shas, as part of the Mesoras HaShas, originally by Rav Yeshoshua Boaz and later emendations by Rav Yeshaya Pick. That's what Oz Vehadar is referring to. They split up the gemarra citations and the marginal notes, which used to all be in the margin. Now gemarra citations are up top and marginal notes were moved to the hagahos vetziyunim.
Note: you can distinguish which note is from Rav Yeshoshua Boaz and from Rav Yeshaya Pick. The ones from Rav Yeshaya Pick are surrounded by square brackets .