The GPL is a copyright license and copyright only covers a concrete tangible expression. Which would include the content.
If you want to have others learn from or reuse your LaTeX content, split it out in a package and/or document class.
I have created frameworks for typesetting complex documents in LaTeX and negotiated a setup where I split out all of the layout-specific stuff (including proprietary fonts) into a document class kept private to the customer and all the general mechanisms into a number of packages released as Free Software (under the GPL, incidentally). I have had contact with the actual documents processed in that manner just for the sake of debugging.
LaTeX makes it comparatively straightforward to split material into different licensing domains appropriate for the kind of intended reuse, making the actual "document" contain basically only content.
If you split out your work along such lines, licensing the different parts/aspects under different licenses becomes feasible, with the conceptual "content/structure" split actually being traceable to separate files and copies.