Probably someone has already noticed this but I gave the gtk_args macro a try and noticed it's missing the inputs tag.  To get it to work I needed to do this:

class Dungeon
  gtk_args
  attr_accessor :inputs
  ...
end
$dungeon = Dungeon.new
def tick args
  $dungeon.args = args
  $dungeon.inputs = args.inputs
  $dungeon.tick
end

Noted. I'll make sure that's fixed in the next release.