Well, not even today's fancy laptops work well outside or in sunny places. You might be able to find a goofy old sun shield they used to sell for the Gameboy... while it won't clip onto the PCE GT, you might find a way to tape it on (or use a rubber band). The shield has three sides, and usually the vertical sides are tapered...
Actually, it coudn't hurt to make a ghetto sunshield out of cardboard and see if it helps, then try ordering a plastic GB shield.
Otherwise, get a parasol!