1. You think that Bunta would fight against kid?
2. Summons are usually used when you are outnumbered, when opponent also have summon, so that make summon vs summon and when you fighting opponent a lot stronger than yourself. Sasuke and Naruto was around equal so....
Then Naruto could just summon gamakichi first and then gamabunta.Oro used summon against naruto even though he was stronger and naruto didnt have summon.Kakashi used summon against zabuza.Jiraiya used summon against ebisu.I dont see why naruto wouldnt summon bunta.Kishi and his plotholes i guess..