The editors had to work from Heidegger's often handwritten manuscripts, but they had leeway to include various student transcriptions in the published editions.
They were given strict guidelines which are generally noted in the Editor's Afterwords (Nachwort des Herausgebers).
It appears that the outline of the majority of the published writing were selected by Heidegger himself.
The Gesamtausgabe is divided into four series: Each series is divided as follows: (The publisher Vittorio Klostermann points out that one notebook is missing from early 1930s the Nachlass: "Überlegungen I" or "Winke Überlegungen (I)" is still missing (GA 97 p. 521 editor note).
A contents list with details of all English translations of Martin Heidegger's works (including those currently in preparation) is available.
Vittorio Klostermann's editions for Martin Heidegger's collected writings the HGA prospectus 2021 in .pdf format is online here.
Heidegger: Bibliography of addresses and courses he took and taught: German and English.
Martin Heidegger Esoteric Writings: An Index https://archive.org/details/HeideggerEsotericIndex Ferrer, D.F (2017).
Heidegger Anmerkungen I-V (Schwarze Hefte 1942–1948): An Index https://archive.org/details/HeideggerGA97Index Ferrer, D.F.