This is not a serious encounter, it's not a life or death situation. The only thing we can conclude from this is that Gear 5 Luffy with Zoro is too much for Gaban, which is obvious. Why continue the fight when the outcome is known, Gaban got the information he needed, Luffy is worthy of the key. It could also be that he stopped because of the HK's arrival. You're jumping the gun here and are going to realize that once Oda has Gaban show more of his strength in future chapters.