Gauss' Law for magnetism applies to the magnetic flux through a closed surface. In this case the area vector points out from the surface.
Because magnetic field lines are continuous loops, all closed surfaces have as many magnetic field lines going in as coming out. Hence, the net magnetic flux through a closed surface is zero.
Net flux = ò B · dA = 0
Gauss' Law for charges was a very useful method for calculating electric fields in highly symmetric situations. Gauss' Law for magnetism is considerably less useful, but we'll come up with another law that enables us to calculate magnetic fields in highly symmetric situations.