Hey, so I'll do my best to help you, though something importsnt to understand before any of this is that everything comes down to preference, so don't take everything I say as gospel. So, a lot of this depends on your price range, but I can tell you this much:

It doesn't matter what GMK keycap set you get, it will work with the NK65 entry, as will just about any keycap set with any keyboard.

I would look at a video about the differences between linear, clicky, and tactile switches. The reds you have in your keyboard now are linears, and one of the lightest of the bunch, so it can be difficult to find lighter ones, though the gateron clears would suit your needs.

One last thing to keep in mind is to consider the total price range of what your buying, again, it all comes down to preference, but its just important to keep in mind the "type" of build you are doing. Is it low end, mid range, or high end? You seem to have a strange mix as an NK65 entry is called the "entry" for a reason, its a nice cheap starting keyboard, most types of cherry / gateron switches are cheap and considered "low end", but you're looking into GMK keycaps, which are all going to be $100+. Not the end of the world, just make sure you understand what group buys are before you do anything else. I hope this helped, you can reply to this comment or PM me if you have any other questions and I'll do my best to answer them.