The CategoricalGibbsMetropolis step method is the default for the Categorical distribution. The Metropolis step method may not work for Categorical in some circumstances, e,.g. as part of compound distribution. However, CategoricalGibbsMetropolis currently can only be used with Categorical or Bernoulli distributions.
I have a model where I need to use a DiscreteUniform distribution rather than Categorical, as the k parameter can be very large. It would be useful if the CategoricalGibbsMetropolis step method could also be used for DiscreteUniform distributions, which are after all a special case of Categorical.