$$\require{cancel}$$
The field above an infinite uniform plane lamina of surface density $$σ$$ is $$−2 \pi Gσ$$. Let $$\text{A}$$ be a point at a distance a from the lamina and $$\text{B}$$ be a point at a distance $$b$$ from the lamina (with $$b > a$$), the potential difference between $$\text{B}$$ and $$\text{A}$$ is
$ψ_{\text{B}} - ψ_{\text{A}} = 2 \pi G σ (b-a). \label{5.8.14} \tag{5.8.14}$
If we elect to call the potential zero at the surface of the lamina, then, at a distance $$h$$ from the lamina, the potential will be $$+2 \pi Gσh$$.