Well, what if I generate some nice GPT-based content on my personal computer, look at it on the screen, and then re-type it by hand on my "protected" smartphone?

All you end up doing is implementing another ham-fisted DRM scheme that ultimately accomplishes nothing but makes the pointy-haired bosses happy. Solving social problems like this with "moar technology" is, in the general case, not a recipe for success, and I don't see why this would be any different.