Fogging is a real problem with GoPro cameras, particularly in diving where humidity is so often an issue. Closing the camera in a damp tropical environment, then submerging the camera in cooler water to dive a tropical reef is a sure recipe to fog the inside of the optical window.
I understand the desire to keep the case as small as possible. particularly for an action cam. But why? Why could GoPro not include enough room for a small desiccant package, a few millimeters would have been enough. Yes, there are the little pieces of blotter paper sold by GoPro and others, but these are not nearly as effective at removing moisture from the case as true silica gel desiccant products that absorb far more moisture per volume. I have seen GoPro cameras fog up even when properly used with the Anti-Fog inserts.
There is plenty of room in my other camera cases for desiccant packs. I keep a good pile of desiccant on hand in nice little packets just perfect for loading into the camera cases. I put the used packs in my toaster oven for an hour at 150°F to bake the moisture out and reuse the packs over and over again. Simply store in a tightly sealed, airtight container for use.