Here is another example to think about:
If you write a document in Microsoft word, who does that document belong to, you or Microsoft?
Obviously, the content of the document is copyright you.
Similarly, although the Moodle code is copyright, and you can use it under the GPL, that does not affect the copyright of content in your Moodle.
Of course your question goes beyond that, and other people have answered other aspects of it. If you have never done so, you may find reading the FAQs at the Free Software Foundation helpful.