Hm... I see two factors to this. First of all, just as Feldis said, Gollnir saved his students with a smile, time and time again. Just like him, Kintoki is fighting to save humanity and doing so with a smile. In this moment, Kintoki truly embodies Gollnir's belief in the future generations and the value of facing things even if they're difficult.
But besides that, Odin also isn't who he once was. Something that Gungir is aware of, I believe. And this fight reawoke a side of Odin that he had cast off eons ago. Kintoki's resolve is resonating with those remnants of who Odin used to be, however much he might try to deny it. And perhaps Gungir, having been Odin's single oldest companion, is acting on an opportunity to save Odin from the influence of the Primordial Gods?