  Let $\mathfrak{g}$ be a simple complex Lie algebra of classical type. A
generalized Verma module induced from a one-dimensional representation of a
parabolic subalgebra of $\mathfrak{g}$ is called a scalar generalized Verma
module of $\mathfrak{g}$. In this article, we use Gelfand-Kirillov dimension to
determine the reducibility of scalar generalized Verma modules of
$\mathfrak{g}$ associated to two-step nilpotent parabolic subalgebras of
non-maximal type. Such a module exists only when
$\mathfrak{g}=\mathfrak{sl}(n,\mathbb{C})$ or $\mathfrak{so}(2n,\mathbb{C})$
for classical types. We find that the reducible points of these modules can be
drawn in a two-dimensional complex plane.
