Me three - I was the same way. I had no trouble understanding the work or memorizing things, so my attitude toward homework was exactly as you describe, “Jeez, I did fine on the test. Can’t you see I get it, already?” Here’s what I’d tell your kid:
As I got older, I discovered that the point of homework is not learning, but overlearning. It’s intentionally repetitive, so that the knowledge becomes automatic. It’s great to be able to figure out how much 16 X 16 is, but it’s so much better to just instantly know, without having to think about it at all, that it’s 256. The only way to have that happen, unless you’re a savant, is to practice it over and over. Also, the more deeply you ingrain a skill, the longer you will retain it, and the easier it will be to relearn if you do lose it. I still remember being that age and feeling that learning was almost effortless, and that once I learned something, it was mine forever. I wish someone could have warned me that these things wouldn’t always be true.
Another reason homework is especially important for gifted kids is that so many things come so easily to them that they often just give up when they don’t enjoy something, or don’t understand it right away. Homework helps teach them strategies for learning when it doesn’t come easily. No matter what they do in life, and no matter how brilliant they are, there will always be things that take extra effort. At your kid’s age, refusing to learn something just because it’s not your favorite thing is really limiting yourself. I love my work now, but I would have certainly had a different career path, and would be much farther along in it, if I had tried to keep my options open and do my best in everything, rather than just the things I liked in high school.
And finally, when I was young, I was a smug, superior little snot. I was smart, and I knew it. I felt pity for the kids who didn’t just pick things up instantly like I did. But the other side of that coin was that when there was something I didn’t understand or couldn’t do on the first try, it wasn’t just that I wasn’t interested - I felt like an abject failure. My sense of self-worth (and the worth of others) was completely tied to “natural” ability. Innate skills seemed real, while something I had to learn seemed fake, somehow. I felt ashamed of having to work at something, almost as though I were an impostor: I couldn’t “really” do X; I had to learn how. Meanwhile many of those kids I pitied felt no shame in working ten times harder than I did, and in a few years, they were making up the difference. By the time I got to college, I could no longer just skate by on my natural ability, and found myself floundering, while “dumb” kids who knew how to study were handing me my ass. Their knowledge and skills were as real as they come.
Ultimately, it’s kind of like discovering you have a super-power: sure, you can do some cool stuff with it now, that a lot of other people can’t do. But if you use it as a starting point, you’ll go beyond your wildest dreams. And if you don’t use it, you’ll lose it.

