為什麼 gamma wave 能穿過金屬？

It is because of the high energy of gamma ray that making it to be an ionizing radiation that demonstrates its particle nature.

Radiowaves are electromagentic waves of much lower energy. For example, the microwave emitted by a 300 GHz radar is of order of 10^-22 eV. At such low energy, radiowaves (or microwaves) possess the property of waves. When radiowave passes through a metallic sheet, the electric field of the wave causes free electrons to oscillate and thus absorb energy from the wave.

Ionzing radiation, like gamma ray, has much higher energy (1 MeV or more). In such high energy, gamma ray behaves more like particles (photons). Apart from causing ionization from collision with target atoms when the gamma ray photons passing through metal, most of the gamma ray photons are able to pass through the empty space between atoms of the metal.

In fact, at energy range of visible lights, the dual nature of waves is exhibited. Light behaves on one hand as waves, as in interference and diffraction. But on the other hand, light also behaves as particles, as in photoelectric effect. The particle nature is more prominently demonstrated at the high energy end of the electromagentic spectrum. Hence, x- and gamma- rays behave more like particles than (eectromagentic) waves.