We introduce unique sink orientations of grids as digraph models
for many well-studied problems, including linear
programming over products of simplices and generalized
linear complementarity problems over $\P$-matrices (\PGLCP).
We investigate the combinatorial structure
of such orientations and develop randomized algorithms for finding the
sink. We show that the orientations
arising from \PGLCP\ satisfy the combinatorial
\emph{Holt-Klee} condition known to hold for polytope digraphs, and
we give the first expected linear-time algorithms for solving \PGLCP\
with a fixed number of blocks.