as far as Valinor goes, the Ring(s) and all their works were purely
Middle-earths works, and for Middle-earth to sort out. As I've read it,
the Ring bearers are allowed to go to Valinor only as a special dispensation
for standing up to the forces of Darkness so bravely and to heal their souls of
the damage done to them by bearing Saurons Ring. This would be
particularly true of Frodo, but also to a lesser degree for the other
Hobbits. I therefore don't think it was simply because they bore the
Ring.
Gimli, I'd guess was allowed for two linked reasons. Firstly as a
representative of the Dwarves who had also bravely stood up to Sauron (though
off stage from the main text of the Lord of the Rings) and who the Valar may
have then wanted a closer understanding of, and secondly specifically Gimli by
the intervention of Galadriel.
Finally I don't think simply membership of the fellowship entitles leave to go
to Valinor, (in which case why not Aragorn at the end of his life?). As
far as Boromir goes, he has has died, and his soul would have gone briefly to
the Halls of Mandos, then left Middle-earth, even if bodily resurrection of Men
were possible. As far as I know there is only one example in Tolkien of
ressurection in Men happening, where Beren comes back. This though was a
very special case. I think he actually aquited himself quite well,
the evil, and overwealmingly powerful, influence of the Ring playing on his
fears for his people, and doubts about the wisdom of entering Mordor count as
extenuating circumstances for the incident on Amon Hen in my
eyes.
