Will you do a tutorial for this or you know where can I find one?
I just went to figure out how to do it and it's ridiculously easy. I've already made several playable myself.
Simply browse characters in the resource manager (icons of purple stick figure), click on them, in the left hand panel at the top it will show a CHECK MARK symbol. Click that to be able to edit it and then scroll down to where it says "GM". There are two categories below it. Simply put whatever you want there for where you want to the monster to show up in the GM creatures directory. So easy anyone can do it.
I am going through and literally doing every creature in the game right now. Might take me a couple hours, but should be finished before tomorrow.