To me,
fits in perfectly with the spooky esthetic of the Haunted Mansion. Sure, I'd prefer Uncle D, but Gonzo is a good main character choice as it's a subtle nod to Roly Crump's Museum of the Weird concept that was meant to be part of the original mansion/attraction but got scrapped and left unused until the comic book series.
Something else... This special is meant to take place during Halloween. So does that mean that besides the regular mansion dwellers, might we see some of the Disney Villains congregating outside as part of the stage shows or in the plot itself?
And I'd love this concept even more if the special warranted a sequel set during the DLR's Christmas celebrations for obvious crossover reasons.