Here is the implementation of get_gray_xy that I used:

    static uint64_t deinterleave(uint64_t x) {
        x = x & 0x5555555555555555;
        x = (x | (x >>  1)) & 0x3333333333333333;
        x = (x | (x >>  2)) & 0x0f0f0f0f0f0f0f0f;
        x = (x | (x >>  4)) & 0x00ff00ff00ff00ff;
        x = (x | (x >>  8)) & 0x0000ffff0000ffff;
        x = (x | (x >> 16)) & 0x00000000ffffffff;
        return x;
    }
    static void get_gray_xy(uint64_t n, uint64_t *x, uint64_t *y) {
        uint64_t gray = n ^ (n >> 1);
        *x = deinterleave(gray);
        *y = deinterleave(gray >> 1);
    }

I don't think the gray code solution can work well for sets of different sizes because the area of indexes that you go through grows as a square.

But it does work for different number of dimensions or different number of nested loops. For 3 dimensions each coordinate is constructed from every 3rd bit instead of 2nd.

So if you have index 14, then gray code is 9 (1001) which means in two dimensions x=1,y=2 and in three dimensions, x=3,y=0,z=0.