What is the significance of Gauss' theorem? Is it applicable to any other fields other than electric field?

The application of Gauss's theorem gives rise to an easy evolution of electric field in the case of

The application of Gauss's theorem gives rise to an easy evolution of electric field in the case of

A : Gauss theorem is applicable on any closed surface. R : In order to find the value of electric field due to a charge distribution, Gauss' theorem should be applid on a symmetrical closed surface.

The Gauss' theorem for gravitational field may be written as

The Gauss' theorem for gravitational field may be written as

Are there any sources of magnetic field other than magnets?