Grid when close to skybox

So I was playing on some awesome map in gmod and noticed that when u got close to edge of map, a green grid showed up, how do I do this?

I’ve seen this effect used in Empires mod maps. I’m pretty sure it was accomplished with a manually created texture mipmap. The highest resolution of the texture would have the gridlines, while the lower resolution versions would be completely transparent. This would make the texture completely invisible from far away, and show the gridlines when you got up close. This also meant that players using lower texture detail settings would never see the gridlines at all.