The GP32 was never that great in sunlight, whichever version you went for, but it did have surprisingly good visibility in the NLU. AFAIK the biggest problem was not the screen itself, but rather cover over it - there was a time when the standard solution was to replace the plastic cover with a glass one, which aparrently greatly boosted visibility in daylight.
NLU and FLU would both have had pretty much the same visibility since FLUs were converted from NLUs. In contrast, BLUs had a completely different screen which was iirc unsurprisingly not so great in sunlight, though I can't remember why.
So:
NLU/FLU = not so good in sunlight (though fine elsewhere).
NLU/FLU+Glass screen = decent.
BLU = not so great.
I think. Though it's been a while since I used mine.