  The electric structure of twisted bilayer GeSe, which shows a rectangular
moir\'{e} pattern, is analyzed using a $\bm{k}\cdot\bm{p}$ type effective
continuum model. The effective model is constructed on the basis of the the
local approximation method, where the local lattice structure of a twisted
bilayer system is approximated by its untwisted bilayer with parallel
displacement, and the required parameters are fixed with the help of the
first-principles method. By inspecting the twist angle dependence of the
physical properties, we reveal a relation between the effective potential under
moir\'{e} pattern and the alignment of the Ge atoms, and also the resultant
one-dimensional flat band, where the band is flattened stronger in a specific
direction than the perpendicular direction. Due to the relatively large
effective mass of the original monolayers, the flat band with its band width as
small as a few meV appear in a relatively large angle. This gives us an
opportunity to explore the dimensional crossover in the twisted bilayer
platform.
