"...and he was gathered to his ancestors"

"and he expired, and died, and was gathered to his fathers"

In the case of Abraham, as with several others figures in the narrative of Bereshit, Scripture is not content to simply say "he died": it must re-iterate the point in three times. Welsh bards liked things in threes, which is why the triad is one of the most typical forms of Celtic literature:

Three are the fathers--Abraham crosser of rivers and father of nations;
Isaac willingly bound and fearer of G-d;
Jacob who wrestled and wept for his son and blessed tribes.

But the Author of the Torah is not a Cymric poet: the triple usage is there to teach us something. So why does the Torah use this triplicity to tell us that Abraham and the others shuffled off their mortal coil?

None of the sources I have been able to consult in the last few days, since my attention was attracted to this point, have anything to say directly in answer to the question. Rashi says that "expired" is used in connection with righteous persons, but this can not be a strict rule: otherwise we would have to infer, for instance, that neither Rachel nor Joseph were righteous, because the term is not used in connection with their demises. They simply die. The same with Moses.

Further investigation reveals that the word translated into English as "expire" does not really mean "die"; it refers to the ceasing of toil and labor, to being worn out: we have here expire in the meaning of expiration date on a carton of milk. It is not death itself that is meant, but the ending of work and labor, and the ending (since we are said to be referring to the righteous) to that person's good deeds and charity. It is not only Abraham's life that ceased, but his hospitality as well. (Although we may presume that Isaac kept up his father's open tent policy.) His good deeds cease; his righteous toil and labor have come to their end.

Then we get to the physical death, in the case of Abraham elaborated with the statement that it was in a good old age and full of years. And then comes the statement that "he was gathered to his fathers". The generic gloss on this phrase is that it denotes spiritual immortality--he joins the ancestors in Heaven. But it can also be considered in the sense that he becomes one of the ancestors. The deceased becomes part of our heritage, and his righteousness (since we are speaking of a righteous person) is among the influences, examples, and precedents by which our own lives are shaped. The lovingkindness and good deeds of Abraham himself are no longer here, but we can carry on the work of lovingkindness and good deeds which he began, and maintain the tradition of lovingkindness and good deeds which he started.