Codes over Galois rings have been studied extensively during the last three decades. Negacyclic codes over $GR(2^a,m)$ of length $2^s$ have been characterized: the ring $\mathcal{R}_2(a,m,-1)= \frac{GR(2^a,m)[x]}{\langle x^{2^s}+1\rangle}$ is a chain ring. Furthermore, these results have been generalized to $λ$-constacyclic codes for any unit $λ$ of the form $4z-1$, $z\in GR(2^a, m)$. In this paper, we study more general cases and investigate all cases where $\mathcal{R}_p(a,m,γ)= \frac{GR(p^a,m)[x]}{\langle x^{p^s}-γ\rangle}$ is a chain ring. In particular, necessary and sufficient conditions for the ring $\mathcal{R}_p(a,m,γ)$ to be a chain ring are obtained. In addition, by using this structure we investigate all $γ$-constacyclic codes over $GR(p^a,m)$ when $\mathcal{R}_p(a,m,γ)$ is a chain ring. Necessary and sufficient conditions for the existence of self-orthogonal and self-dual $γ$-constacyclic codes are also provided. Among others, for any prime $p$, the structure of $\mathcal{R}_p(a,m,γ)=\frac{GR(p^a,m)[x]}{\langle x^{p^s}-γ\rangle}$ is used to establish the Hamming and homogeneous distances of $γ$-constacyclic codes.