hw do you know his base speed is so high?,gaara dosent need to be fast,his sand is what is very fast. He was able to make a village sized shield in the split second it took for an explosion to go off,thats blistering fast.
Sand is a very small particle actually,a sand coffin consisits of numerous of these particles clumped together. So a kunai pf minato can only touch the sand grains directly in contact with it. A sand tsunami isnt one solid object,a kunai of minato can only touch some grains of sand nd teleport that. A kunai,nor miato wont touch all of the sand grains at once,
Bunta wont power out if gaara aims for his legs,didnt you see the size of the sand arms gaara made in the last chapter?. Gaara can also decide to use sand spikes instead which will just pierce bunta from below.
A jumping bunta will only make a good target for a flying gaara,who can just move away from an oil spit.
It was base speed that saved kid Kakashi when he was about to be killed, Minato was way by were Rin and Obito was 1 second and saving Kakashi while also marking the ninja with his mark all in 1 move. Then he drops his bag and FTG to the guy
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Then Moving from where Kushina was to saving Baby Naruto when Obito was right about to stab him is base speed
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and Obito said he lives up to the Yellow flash nickname.
I know of Gaara's feats, I have say Gaara can beat Itachi and MS Sasuke in some fights. I don't say low diff but mid to high depending on who he's fighting. Gaara's not beating Minato low difficulty. Also when the sand is used to attack or defend, Gaara has it in a mass form of some kind. All the sand are control via chakra that Gaara use to move it, make it denser or make it less dense. When the sand is used for attack or defense then all the sand is touch another grain of sand so yes Minato can FTG it away it all away since it's clump together and all touching. Minato is a Genius that Jiraiya held above all even with people like Shikaku in Konoha he has the smarts, annalistic skills, speed, and right tools to use in the fight, also he is said to never make a wasted move.
He doesn't have to touch all the grains, each grain is touching each other and that's all Minato needs to FTG all of it away. Example, Sasuke and Naruto = grain of sand that were connected from the chakra arm, Minato FTG both of them because they were connected so same thing with the grain of sand touching.