Why are gamma rays not affected by a magnet as they pass over it?

Only alpha and beta ray will be affected because they are charged with positive and negative ions respectively. In Electric field, they will be moved(i cant figure out a suitable word). THey move in a hyperbolic path and follows the positive and negative terminal. Both the alpha and beta will flow to opposite to each other where alpha will move to the negative terminal and beta will move to the positive terminal.  In magnetic field, its more complicated then electric field. U eva heard of the Left Hand Flemming Rule?, Its hard to explain without a diagram. Maybe u shud search out urself. They will follow a circular path.  In both fields, alpha will move less while beta will shift more because alpha is heavier then beta.

As i say just now, gamma is not affected by both fields because it isnt charged

Were is the question