My understand of the GPL is the following:
Anything you build utilizing existing Discourse APIs is yours to do with. You can keep it closed source (granted it would be difficult to sell it and not provide the source to the client buying it), sell it, whatever.
If you need to alter the Discourse core code, that is where things get sticky. You are modifying code that is under the GPL, therefore all modifications you make should be made publicly and sent as a PR back to Discourse so they can include them in future releases (this benefits you as a plugin developer too – as you are making sure that Discourse by itself can support what you feel is necessary for good plugin development).
Your plugin can still remain closed source (you don’t need to make it publicly available, for free – you can definitely charge for it) even if it utilizes new API features you needed to add to Discourse which you plan to submit back to the Discourse project.
Hopefully that all made sense. But I believe that to be what the GPL means and should be interpreted as. I never followed any of the WordPress conversations, so I won’t get into that, but if they stood on a different side than mentioned above, I feel they may be wrongly interpreting the purpose of the GPL license (or I am).