Fujifilm X-A3 has a 24.0MP APS-C (23.5 x 15.7 mm ) sized CMOS sensor and features EXR Processor II processor. On the other hand, Panasonic GM5 has a 16.0MP Four Thirds (17.3 x 13 mm ) sized CMOS sensor and features Venus Engine processor.
Fujifilm X-A3's sensor provides 8MP more than Panasonic GM5's sensor, which gives a significant advantage in real life. You can print your images larger or crop more freely.
On the other hand, please keep in mind that Max sensor resolution is not the only determinant of resolving power. Factors such as the optical elements, low pass filter, pixel size and sensor technology also affects the final resolution of the captured image.
Below you can see the X-A3 and GM5 sensor size comparison.
As seen above, Fujifilm X-A3 has a 1.6x Larger sensor area than Panasonic GM5. Larger sensors give photographer more control on the depth of field and blurry background compared to smaller sensor when shot in same focal length and aperture.