He is called Mountain Eater because of what He done to protect Roger when He wasn't able to Fight,
Gaban knows how Important it is to have a Crew Member who can handle Level of Fights that wd usually require the Captain just in case things go south, similar to how Luffy just entrusted Zoro to continue the Fight against Kaidou/BM while He was Resting Post-Gear 4 or when He let Zoro take on Fujitora so He can continue on his way ... etc
Gaban's Question was Clear, He wondered what would Luffy do if Loki wasn't Cooperative,
He got his Answer once Zoro joined & teamed up with Luffy, He knew that Luffy not only have Someone who can Fight in his place, but They are also willing to Team-Up against a Powerful Foe,