Both Nikon P900 and Panasonic GX85 have 16.0 MP resolution sensors but Nikon P900's sensor is (1/2. ) and Panasonic GX85's sensor is Four Thirds (17.3 x 13 mm ).
Since Panasonic GX85'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 Nikon P900.
One other difference between these two cameras that is worth mentioning is that Panasonic GX85's sensor doesn't have an anti-alias (Low-Pass) filter. Removing anti-alias filter increases the sharpness and level of detail but at the same time, it increases the chance of moiré occurring in certain scenes.
Below you can see the P900 and GX85 sensor size comparison.
As seen above, Panasonic GX85 has a infx Larger sensor area than Nikon P900. 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.