Mike,
I have both a GK1 Ver. 1 and Ver. 3. I also have several Reflex Sights. I am not going to recommend a brand because it is not definitive. I have now also shot thousands of rounds through these guns and as stated before in other threads, I shoot every day. I shoot in many lighting conditions from very low light to bright sun. So what is important is wide field of view for quick target acquisition, 2mm dot diameter so not to block your target at longer distances, highly polished coated glass to prevent dot flaring at higher intensity and many different LED intensity levels. Just as important as the sight specs is the height above the barrel center because it effects the amount of hold over and hold under required at different distances. The lower the sight resides in relation to the barrel center the better,
That said, Those sight specs are hard to find that mount to the 11mm rail directly. I know of NO sight with those specs that mount to the 11mm rail without an adapter, but all adapters require additional height above the barrel center except the type that telescopes sideways occupying the width difference between the Picatinny 22mm standard and the 11mm rail. This style offers No height change. Lastly, mounting any sight over the receiver offers No accuracy advantage only aesthetics. If you look carefully, you can find a sight that matches these requirements on AliExpress at a low price.