This question is too broad.
The problem of understanding how the multiplicative properties of $n+k$ relate to that of $n$ is an extremely difficult problem, and you might argue that this is the reason behind the difficulty of some of the most difficult problems in number theory. Obvious examples include Collatz, Goldbach and the twin prime conjecture. Indeed, (in some sense) most progress on these problems come down to addressing this chief issue of understanding the multiplicative properties of $n$ and $n+k$, in a round-about way. So, looking into the extremely broad literature on this subject can help.
As far as I know nothing nontrivial is known that is elementary. The obvious $(n,n+k)=(n,k)$ is known, for example, and we can use this to see that $n,n+k$ can't share any common prime factors except those dividing $k$. Beyond this simple observation, much of the rest is research level maths.