Dear Katsushi Waki,
to compute the cohomology of a pc group G and a module M, the
generating matrices in M must correspond to the action of the
elements in Pcgs(G). In your example you can achieve this by:
gap> g := GL(2,2);; gap> orb := Orbit(g,One(GF(2))*[1,0],OnLines);; gap> iso_pgl := OperationHomomorphism( g, orb, OnLines );; gap> pgl := Image( iso_pgl );; gap> iso_g := IsomorphismPcGroup(pgl);; gap> G := Image(iso_g);; gap> mats := List(Pcgs(G), x -> PreImagesRepresentative(iso_g, x) );; gap> mats := List(mats, x -> PreImagesRepresentative(iso_pgl, x) );; gap> M := GModuleByMats( mats, GF(2) );; gap> TwoCoboundaries( G, M ); [ [ Z(2)^0, 0*Z(2), Z(2)^0, 0*Z(2), 0*Z(2), 0*Z(2) ], [ 0*Z(2), 0*Z(2), 0*Z(2), Z(2)^0, 0*Z(2), 0*Z(2) ] ] gap> TwoCocycles( G, M ); [ [ Z(2)^0, 0*Z(2), Z(2)^0, 0*Z(2), 0*Z(2), 0*Z(2) ], [ 0*Z(2), 0*Z(2), 0*Z(2), Z(2)^0, 0*Z(2), 0*Z(2) ] ] gap> TwoCohomology( G, M ).cohom; ZeroMapping( <vector space of dimension 2 over GF(2)>, ( GF(2)^0 ) ) Best wishes, Bettina