Amplification through ionization
Along its path, a charged particle (top) snatches thousands of electrons leaving behind as many positive atoms (ions). The presence of an electric field allows to collect all these positive and negative electric charges on an anode and a cathode. The collection of so many charges generates a measurable electrical signal. In the case of a neutral particle (below) the particle do not ionize directly. The gamma of the lower figure would leave no trace, until it set in motion an electron (A), and later a second one (B) which will ionize. There is also a signal due to these secondary electrons.