  Let $X$ be a normal arithmetically Gorenstein scheme in ${\mathbb P}^n$. We
give a criterion for all codimension two ACM subschemes of $X$ to be in the
same Gorenstein biliaison class on $X$, in terms of the category of ACM sheaves
on $X$. These are sheaves that correspond to the graded maximal Cohen--Macaulay
modules on the homogeneous coordinate ring of $X$. Using known results on MCM
modules, we are able to determine the Gorenstein biliaison classes of
codimension two subschemes of certain varieties, including the nonsingular
quadric surface in ${\mathbb P}^3$, and the cone over it in ${\mathbb P}^4$. As
an application we obtain a new proof of some theorems of Lesperance about
curves in ${\mathbb P}^4$, and answer some questions be raised.
