Middle European Mathematical Olympiad 2020 Problem I-1

Let $\mathbb{N}$ be the set of positive integers. Determine all positive integers $k$ for which there exist functions $f\colon \mathbb{N}\to \mathbb{N}$ and $g\colon \mathbb{N}\to \mathbb{N}$ such that $g$ assumes infinitely many values and such that

$$f^{g(n)}(n) = f(n) + k$$

holds for every positive integer $n$.

(Remark. Here, $f^i$ denotes the function $f$ applied $i$ times, i.e., $f^i(j) = \underbrace{f(f(\ldots f(f(j)) \ldots))}_{i \text{ times}}$.)