Evaluating the non-Fourier thermal transport accurately in GaN transistors is very important for its design and applications. Since the width of the heat generation in the transistors is comparable to phonon mean free paths of GaN, phonon ballistic-diffusive transport exists and can significantly affect the heat transport process. In this paper, thermoreflectance thermal imaging is adopted to observe and verify the non-Fourier thermal transport. A series of samples with different Au heater sizes on a wafer consisting of a GaN film grown atop SiC substrate are designed and processed. Temperature field distribution of the heat source is observed with the advantage of high spatial resolution of thermoreflectance thermal imaging, and the hot spot temperature of the sample is measured directly. Phonon ballistic transport makes the hot spot temperature and the corresponding thermal resistance higher than the prediction of Fourier's law. The smaller width of the heater lines, the stronger the ballistic effect, and the greater the deviation from Fourier's law. The experimental results are compared with a hybrid phonon Monte Carlo-diffusion simulation considering the cross-scale ballistic-diffusive regime. The variation of the thermal resistance ratio of the simulated sample with the heater width is similar to that of the experiments, which further verifies the non-Fourier thermal transport phenomenon of the GaN device observed in the experiments.