Appologies if I misunderstood your question. A sketch could help understand what you want to do.
I assume VEC_Gc is the cross product of the other vectors.
Your vector is defined by direction and size, but what is the origin?
If the origin is (0,0,0), then the endpoint coordinates are the same as the values defining your vectors:
VEC_Gu = [40 ; 0 ; 0 ]
VEC_Eu = [ 0 ; 20 ; 0 ]
VEC_Gc = cross(VEC_Eu, VEC_Em)