  Let $A = \prod_{1\leq i\leq n} A_i$ be the product of principally polarized
abelian varieties $A_1, \ldots, A_n$, each defined over a number field $K$, of
a shared dimension $g$, and pairwise nonisogenous over $\overline{K}$. Suppose
that the adelic Galois image of each $A_i$ is open in
$\operatorname{GSp}_{2g}(\widehat{\mathbb{Z}})$. An open image theorem due to
Hindry and Ratazzi gives that there is a constant $c(A_1\times \cdots \times
A_n)$ such that for each prime $\ell>c(A_1\times \cdots \times A_n)$, the
$\ell$-adic Galois representation of $A$ surjects onto a specific $n$-fold
fiber product of $\operatorname{GSp}_{2g}(\mathbb{Z}_\ell)$. We make effective
this result under GRH by proving an explicit bound, in terms of standard
invariants of $K$ and each $A_i$, on the constant $c(A_1\times \cdots \times
A_n)$.
