What I know of the topic:
Avos was originally a five chapter masechta, but a sixth chapter, which is taken from Kallah Rabbati in the Masechtos Ketanos (with some textual variation), was added on later so that there should be six chapters, one for each Shabbos between Pesach and Shavuos. Though the sixth chapter is technically not part of Maseches Avos, it is generally treated as thought it was: MishnaYomis programs always include it and it's included in all standard version of the Mishna and Talmud Bavli.
There are two versions Pirkei Avos, the one that appears in the standard Shas (in the last volume of Seder Nezikin) and the one that appears in the standard siddur (usually near Shabbos Mincha). For the most part the differences are just the way the Mishnayos are broken up (which often leads to confusion when citations in Pirkei Avos appear to be erroneous because they are from the other version than the one being used) but there are also a few differences in the text itself.
Avos d'Rabbi Nasan, which is usually printed along with the Masechtos Ketanos (though my understanding is that historically it was written long before the others) is a midrash that is closely related to Avos and seems to be the closest we have to a "Gemara" on Avos (which otherwise is the only Masechta that does not have a Bavli, Yerushalmi, or Tosefta). However, scholars are unsure if Avos d'Rabbi Nasan is a commentary on Avos or if Avos is actually a summary of Avos d'Rabbi Nasan (or perhaps they both indpendantly stem from an original lost work or tradition).
There are two versions of Avos d'Rabbi Nasan: the one that appears in the standard Shas and one that was published from manuscripts by Solomon Schechter (I know the Artscroll Rashi sometimes quotes the second version as a source for midrashim/interpretations that don't appear in the standard version of Avos d'Rabbi Nassan).
[Pirkei d'Rabbi Elazar is unrelated]