This is just my theory, I could be wrong and I'm not claiming it ru be fact. Just a little disclaimer.
I believe it was Jiraiya who stated that in order for Naruto to summon, he needs to completely empty his chakra, so he can start using the nine tails chakra. This was true when he fought Gaara too, because he lost all his personal chakra against Neji.
When he was going to retrieve Sasuke, he hardly used that much. He had a rasengan attempt but until he actually got to Sasuke, he only used the Kyuubi chakra. I think that more than likely if he tried to use summoning, he'd summon gamakichi or something.
You do know Naruto recovered some of his own chakra while they were waiting on Sasuke to arrive right? He then unleashed even more chakra that he got from wanting to protect someone he loves and it was blue chakra in the anime so his regular chakra is enough to summon Gamabuta. Plus Sasuke saw Kurama when Naruto unleashed Kurama power when they fought so we know it wasn't Kurama's Kurama's chakra.
Naruto got even more chakra from Kurama in his initial chakra mode since Naruto eyes turn to a Kurama's, fangs and claws while Naruto couldn't when he used it before. Naruto had the chakra to summon Gamabuta but he didn't because of plot, Sasuke needed to leave for plot sake.