I also have a gray horse, he's 9 this year and I've never heard of eye issues related to the color. It is possible for some to get melanomas around their eyes but that's it.
Unfortunately, melanomas are one of those things that an owner of a gray horse will usually have to deal with eventually. I had 2 fairly large ones taken off my guy the year before last and he has several more small ones. Most of the time, they are just an aesthetic issue because many of them are just on the surface. I am very particular about checking my guy every few weeks to see if any of them have gotten bigger or if there is any more. The ones that I had removed grew large very quickly but these smaller ones that he has now have not grown much, if at all, since they first popped up.
We had a gray mare years ago that was healthy up into her 30's even though she had a lot of melanomas. Most gray horses will die of old age long before the cancers become life threatening.
He's a good looking guy.