1) he can use ftg and 8 gates but he was 72 years old and he doesn't have enough chakra to perform it
2) minato learned his jutsu from his predecessors.
hiruzen is jiraya'sensei and jiraya was minato's sensei .
Hiruzen at his primes mastered all konoha jutsu before minato was even born.
1. I mean, why doesn't he uses FTG and 8 gates now? He has chakra levels to do so.
2. Start reading managa agin. You missed a couple of things.
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Read the last panel. This was the death Yondaime spoke of. So, you believe mangafacts, right? Then you belive Minato taught him this. Also in the anime it's clearly said, I'll have to use the Fourth's jutsu.