If the magnetic monopoles were to exist, how would the gauss's law of magnetism get modified ?

Gauss law of magnetism states that the divergence of a magnetic field is zero whereas divergence of electric field is not zero. This suggests that magnetic monopole does not exists. Divergence of a physical quantity gives us the idea about diverging nature of a physical quantity. Now, since magnetic field lines are in closed loop, therefore, divergence of magnetic field comes out to be zero. Divergence involves higher mathematics and will be taught in your graduation.

Magnetic monopole is a hypothetical magnet which has one of the pole only i.e. either north or south. If it exists then the Gauss law of magnetism disobeys and then as per the gauss law electric field the divergence of magnetic field is proportional to the magnetic charge density. But this is just an idea till now no magnetic monopole has found.