Coby is not Garp to Luffy's Roger.
Wake up.
Actually no. He is more than garp will ever be actually. Oda has already layed out the foundation
He is trained by the person known through the entire history of one.piece as the greatest marine
Zoro future greatest swordsman and luffy future pirate king stated that coby will be that strong to the point that zoro pointed out that he firmly believes that luffy just created a new powerful threat moving forward because of it.
Just like luffy and zoro coby was the original 3 and was shown in the very first chapter of one.piece with luffy.
During the time skip all 3 of them were trained by legends (greatest marine, dark king of the former pirate kings crew, world greatest swordsman)
He is now apart of a secret organization that's the polar opposite to cipher pole in a matter of 2 years along with being trusted enough in becoming a marine HQ captain capable of fighting and engaging pirates in the new world something pretimeskip luffy stated that his crew were unable to do PREtimeskip and needed to further train for
Was also classified as a hero based on his handling and performance with the rockie port incident, the same incident orchestrated by law which made him a warlord
During marine ford when he stood up to akainu every single pirate witnessed him and stopped in awe just to witness him being extremely bold. Added with the fact that even shanks commended him for his act of bravery and trusts that coby will make a name for himself.
So if you are of the belief that coby won't be great you are actually retarded especially based on the evidence given by people of great importance in this verse in reference to him. So he is actually going to exceed garp based on his current trajectory