How is a "Goses" (someone who is about to die) defined?

Question

In Erchin 1:3, the Mishna opens with:

הגוסס והיוצא להרג לא נדר ולא נערך.

The "Goses" (one who is about to die) and someone who is about to be killed are not used as either a measurement of worth for what their monetary value is or for what their worth according to the values set in the Torah are.
_{(my own translation / elaboration)}

Kehati says:

הגוסס — הנוטה למות

the "Goses" — one who is about to die
_{(translation mine)}

Now, unusually for Kehati, this... doesn't explain much. It's possible that the term is explained more elsewhere, but here I don't see much about how the Goses is defined.

I Googled the term, and Hebrew Wiktionary defined the term as someone on their deathbed with a fatal illness. However, that doesn't answer how the Goses is defined halachically.

How is a Goses defined according to the halacha?

Answer 1

The exact symptoms of a גוסס are not found in the Talmud or Poskim.

A גוסס cannot live for more than three days (See Shulchan Aruch YD 339 that if one hears reports of his relative being a גוסס three days prior, he must assume death and practice the laws of אבילות).

R' Moshe Feinstein (Igros Moshe CM 2:75.5) writes that 'גסיסה ניכר למבינים ורגילין בין החולים' - this stage is recognizable to experts and those who frequent sick people.

R' Shlomo Zalman Aurebach (quoted here) held that nowadays, the laws of גוסס do not apply, as we cannot definitively identify גסיסה.

Answer 2

Rav Moshe Heinemann writes in his sefer on Medical Ethics (8:17),

Goses used to refer to an ill individual who lacked the strength to cough out the phlegm in his lungs (Kitzur Shulchan Aruch 194:1) . You could hear a “death rattle” where every time the individual breathed in and out you could hear the phlegm in the bronchial tubes. That was a sign the person was near death – it could last a few days or just a couple hours. Once one’s breathing is compromised, his life is in mortal danger.

Answer 3

A Goses does not have a life expectancy of more than three days.

Nishmas Avraham Y.D. 339 p. 450 writes that when the author (who is a doctor) asked Rav S”Z Aurbach as to the exact Halachic definition of a Goses, he answered, “You’re the doctor!”

Rabbi David Bleich says that he went through the sugyos of near-death and then when to discuss the practical applications of what is considered a Goses today with Rav Yaakov Kamentsky. IIRC Rav Yaakov told him that "You can talk to me as long as you want but I'm not saying Halacha L'Maaseh because someone who met the Halachic status of a Goses would be long since dead in today's hospitals. "