I don't remember that I thought they said Gin was the strongest of the crew -meaning its not including the captain (Kreig) in the comparison but those who serve under the captain.
Was it online you read that, or in official translated book? We have to be aware some site make bad translations,
Ancient translations in msn groups, and Pirateking.tk original spanish HQ translations.
I could very well remember it wrong but it's clear to me that Gin's convictions are stronger than Kriegs. That makes him a better candidate to captain than Krieg.