GaN etching was performed using planar inductively coupled $Cl_2$-based plasmas and the effects of main process parameters on the characteristics of the plasmas and their relations to GaN etch rates were studied. Also, the GaN etch mechanism was investigated using a Langmuir probe and optical emission spectroscopy (OES) during the etching, and X-ray photoelectron spectroscopy (XPS) of the etched surfaces. The GaN etch rates increased with the increase of chlorine radical density and ion energy, and a vertical etch profile haying the etch rate close to 4000 $AA$/min could be obtained. The addition of 10% Ar to $Cl_2$ gas increased the GaN etch rate and the addition of Ar (more than 20%) and HBr generally reduced the GaN etch rate. The GaN etch rate appeared to be more affected by the chemical reaction between Cl radicals and GaN compared to the physical sputtering itself under the sufficient ion bombardments to break GaN bonds.