Oda clearly showed that time had elapsed between when Gaban dropped into the underworld and the warning he gave Luffy so there was most likely a conversation there. We can tell also from Loki's conversation with Gaban that he was told something.
Not sure why you're so against Gaban having thought them something

. Regardless of what it actually was, there is clear evidence that there was a conversation had. Personally I hope it isn't as simple as you're making out to be. Could very well be that that using a large amount of CoC is one way to go about it but hope it isn't just that.