Welcome,
Sydney! As a moderator, I can give you the Barrow-Downs' official policy. This site is indeed devoted to Tolkien's works, including their various embodiments such as the movies. In order to discuss Peake's works without reference to Tolkien, you would need to find a Peake forum - or start your own, if none exists. However, as
Morthoron said, we do allow discussions - and have had a number of them - comparing Tolkien's works with other fantasy books (
Narnia,
Harry Potter, or Pullman's
Dark Materials trilogy, for example) or discussing the influence Tolkien has had on other authors.
As long as you keep that in mind, you are welcome to begin threads that deal with another author's works in connection with Tolkien. Should things go too far off-topic for our forum policy, one of the moderators or administrators will remind those members involved - no problem, no reason to get upset, standard procedure around here.
The thread
Nerwen mentions is one possible place to look for references to
Gormenghast.
There are also a couple of threads on the various LotR games; a search should find them for you; if not, just post a question and I'm sure someone will direct you there!
Enjoy yourself!