In Gross's paper Heegner points on $X_0(N)$, he considers some examples of general modular curve, not only the usual ones $X_0(N),X_1(N),X(N)$. We know these three kinds of modular curves have "simple" moduli interpretation.

Essentially, Gross considers the modular curves of this kind: Let $K$ be an imaginary quadratic field, $p$ be a prime number inert in $K$. Given an embedding $K\to M_2(\mathbb{Q})$, let $R=\mathcal{O}_K+pM_2(\mathbb{Z})$. Let $\Gamma$ be norm 1 element in $R$. Then my question is what is the moduli interpretation of $\Gamma\backslash\mathscr{H}$, in the similar way as the usual ones?