Short answer
You cannot without any major modifications. The distributed GrailsViewResolver is tightly bound to the .gsp and .jsp extensions.
See grails-core on github for a check.
Long answer
You might be able to create your own Ant task to connect it to the Compile loop of your Grails application, to at least compile your * .md files through GroovyPageCompiler.
This process may look like this (although not quite so, since I rely on the taglib plugin to render in this case, for simplicity).
But this does not solve all your problems. You will also need to register a new permission resolver, and for this you would inevitably go the way of rewriting the GrailsDispatcherServlet .
It looks like your decision to store files in the conf directory might be your best (albeit messy) one. It may take some time for someone to configure GSP file extensions in the future, and this may solve your problem along the way.
I hope you find some useful information.
Daniel Woods
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