Both Sony RX10 III and Panasonic GH5 have 20.0 MP resolution sensors but Sony RX10 III's sensor is 1" (13.2 x 8.8 mm ) and Panasonic GH5's sensor is Four Thirds (17.3 x 13 mm ).
Since Panasonic GH5's has a larger sensor area with the same resolution, this means that it also has a larger pixel area hence better light collecting capacity for a given aperture compared to Sony RX10 III.
Below you can see the RX10 III and GH5 sensor size comparison.
As seen above, Panasonic GH5 has a 1.9x Larger sensor area than Sony RX10 III. 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.