Gradients use the product rule: $\partial / \partial \text{src}[j] = \text{grad}[\text{idx}[j]] \cdot \text{out}[\text{idx}[j]] / \text{src}[j]$, $\partial / \partial \text{base}[i] = \text{grad}[i] \cdot \text{out}[i] / \text{base}[i]$ (the latter requires base[i] != 0).