There‘s really not much that Gaban can accomplish that Rayleigh or Shanks, far more important characters with an established, incredibly strong bond eith Luffy, can’t do far better.
A Roger Pirate acknowledging Luffy at the end of the Grand Line? Is that not the very purpose of returning the hat? Why should we care about Gaban acknowledging Luffy when we’ve already seen Rayleigh and Shanks doing so? Why would Gaban have some important purpose entrusted to him while Rayleigh, who was much closer to Roger, was just lazing around in Paradise?
The only truly interesting thing I can think of for him would be him genuinely believing in Blackbeard, being the Tenth Titanic Captain, and Sanji’s final fight. And that is basically wishful thinking.