Hebrews 9:16-17 Didn't Jesus have to die & someone had to teach us how to live under grace? Why do some say Paul preached another gospel  ?

Hebrews 9:16-17 says

"For where a testament is, there must also of necessity be the death of the testator. For a testament is of force after men are dead: otherwise it is of no strength at all while the testator liveth."

Didn't we need someone to tell us how to live under grace?

Why do some today's Christians say Paul made up his own gospel when Jesus "chose him" (Acts 9:15) to "fulfill the Word of God" (Colossians 1:12) "giving commandments through the Holy Ghost to those He'd chosen" (Acts 1:2)?