The high density plasma clouds with strong magnetic field will appear as CMEs propagating in the interplanetary space. The plasma clouds radiate electromagnetic wave in radio wave band. In order to study the characteristics of CME propagation, the gyrosynchrotron emission mechanism, which is the main radio radiation mechanism of the plasma clouds, is studied. The physical parameter of the plasma clouds (e.g., electron density, electron temperature, magnetic field and energy distribution) inside the CMEs can be derived according to the emission intensity or brightness temperature of them. In this paper, the emission, absorption and polarization characteristics of gyrosynchrotron emission as the optical depth are studied. The gyrosynchrotron emission process of isotropy and anisotropy plasma clouds with thermal and nonthermal electrons are also studied and summarized. Moreover, the expression of gyrosynchrotron emission intensity and brightness temperature is also provided as the refractive index is not equal to 1.