Panasonic GX85 has a 16.0MP Four Thirds (17.3 x 13 mm ) sized CMOS sensor and features Venus Engine processor. On the other hand, Nikon D610 has a 24.0MP Full frame (35.9 x 24 mm ) sized CMOS sensor and features Expeed 3 processor.
Nikon D610's sensor provides 8MP more than Panasonic GX85'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 GX85 and D610 sensor size comparison.
As seen above, Nikon D610 has a 3.8x Larger sensor area than Panasonic GX85. 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.
DxOMark Sensor Scores
DxOMark is a benchmark that scientifically assesses image quality of camera sensors. It scores camera sensors for color depth (DXO Portrait), dynamic range (DXO Landscape) and low-light sensitivity (DXO Sports), and also gives them an overall score. Panasonic GX85 and Nikon D610 sensors have been tested by DxO and the results show that D610 has a better overall score of 94, 23 points higher compared to GX85's score of 71.