What is meant by "ghargarah" (death rattle), and is the illness of death considered a "ghargarah" in which repentance is of no avail, even if it is prolonged?

The meaning of "as long as one does not gargle" in the Hadith refers to the soul reaching the throat, meaning attaining a state after which life is inconceivable. This is prior to seeing the Angel of Death and certain realization of death. Repentance is accepted from a servant as long as their soul has not reached the throat, even if they are in the throes of death, because repentance is not accepted once the soul has reached the throat.