A focus electrode modulation Pierce gun for G band traveling wave tube is designed, with a beam voltage of 20 kV, current of 50 mA, waist radius of 0.056 mm, and a waist position of 10.3 mm. The impact of thermal deformation on the performance of the electron gun is analyzed by using thermal-structure coupling analysis and electron beam trajectory simulation method. In order to eliminate the influence of thermal deformation, an assembly fixer for compensation is designed and verified by experiments. The electron gun has been applied in a variety of G band traveling wave tube, and has solved technical problems of the key component.