Gamabunta is a summon. It's not a technique. That is one thing wrong. Sasuke felt Naruto was gaining strength and he himself had not improved much because his ultimate goal was to defeat Itachi. This was not the fear of Naruto's strength but his own failure, which is WHY he chose to go to Orochimaru. That is like the basics of the basics as far as Sasuke's defection from Konoha is concerned. That is another thing wrong.
Gamabunta cannot be summoned without Kurama's chakra anyway and the fact that Naruto lacked control over his cloak-form and his Chakra control was as poor as it can get - tadpole summons, remember - how does he summon it at that point in time anyway? That is the third thing wrong, and this makes your odds seems very poor.