Its a transflective display so the brighter the Sun, the better it gets.
I had no more of a problem with the Gemini in the Sun than I ever did with the legacy Dynon, G3X, 296, 796 or the GNS/GTN series. It is however much much better than an Ipad or Iphone.
Some of the Gemini units I tried had a mirror shiny slick glass (like an Iphone or Ipad) and some of them had more of a matte finish. The slick ones can act like a mirror in the right situation. The matte finished one was better in this regard. Not sure what the current standard is for what is shipping today.
There are times however that even a transflective display can appear dim. It just depends on the angles of the Sun and shadows and if you are wearing sunglasses or not. Sun right in your face closes down your iris in your eye and tends to turn everything dark in the foreground unless it is also very bright.
On the opposite end, the Gemini does not dim down as low as the legacy Dynon, G3X, 296, 796 or the GNS/GTN series. Once the dimmer voltage gets so low, it will just return to full bright. Be aware of this in the dark!