How can we find an orthonormal basis of $\left\{\nabla p:p\in H^1(\Lambda)\right\}$ for $\Lambda=(0,a)\times(0,b)$?

Question

Let $a,b\in\mathbb R$ with $a<b$ and $\Lambda:=(0,a)\times(0,b)$. It's easy to see that $$G:=\left\{\nabla p:p\in H^1(\Lambda)\right\}$$ equipped with the inner product inherited from $L^2(\Lambda,\mathbb R^d)$ is a $\mathbb R$-Hilbert space. How can we find an orthonormal basis of it?