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We have investigated terahertz (THz) resonant absorption spectra in grating-gate GaN high electron mobility transistors. The results indicate that both the symmetrical plasmon mode and the asymmetrical plasmon mode play an important role in the strong absorption of THz waves. The excitation process and dynamic response of these plasmons are investigated in detail. Our results also indicate that the asymmetrical plasmon is induced by the surface polarization field of the electrodes and the resonant strength of this plasmon is reduced significantly by the decay of the polarization field. Variations in the resonant strength of the plasmonic peaks are consistent with the surface resonant layer model showing that the method we used can be utilized for the study of coupling between THz radiation and plasmons in the channel. In order to achieve wider tunability, more advanced device structures can be explored such as a device that contains double channel layers, in which complicated plasmons can be excited.