Kyuubi has always had healing properties, at least as pertaining to Naruto himself, Naruto on the other hand has always had Uzumaki/Senju Chakra pool and endurance, AND it was noted many times that he could uniquely withstand the Kyuubis toxic hatred poluted chakra.

I think that its a case of each enhancing the other.

It's too early to conclude that the HAchibi could not heal Gai, but it does seem that way, even though Hachibi is only a little bit less dumb than Killerbee, which may be why he never thought to heal Gai on his own, WELL......Kyuubi could have ordered Hachibi to heal him if hachibi could heal him. Makes you wonder.

I'll be willing to bet that the Kyuubi's chakra is unique amongst Bijuus, and not only for healing. I think that Yondaime's decision to make Naruto a Jinchuuriki (As a counter to Madara/Tobi) has more to it than him just merely granting Naruto Bijuu power. There must be something else.

I also get a feling that the Kyuubi was closer to the So6p than the other Bijuus, and the SO6p may have said something to Kyuubi, or granted him something that will play a part in the up coming battles.