This is a quick summary of how best to set up and use Darktable for processing raw files from the Panasonic DMC-GX80 (GX85 in USA).

For reference, I’m using Fedora 28 with the current Darktable 2.6.0 release. I mainly shoot landscapes or architecture, so some of the following may not apply if you’re a portrait photographer.

I was fairly pleased with Darktable for processing my GX80 images - its extraction of detail is particularly good - but found that my results were sometimes a little low in overall contrast and colour images lacked saturation and richness. However, the new filmic module in 2.6.0 appears to mostly resolve these concerns. The other key requirement is ensuring DT has specific camera profile data available for the appropriate aspects of the image pipeline. With these criteria fulfilled, the results from DT can surpass what you can obtain with Snapseed (which in itself is otherwise an excellent app for quickly enhancing in-camera JPEGs).

1. Obtain the DCP profile ‘style’ files for the Panasonic GX80 (potentially downloadable from er… various places or you can install Adobe DNG Converter under WINE to extract them).
   You could also try the Rawtherapee DCP profile for the GX85. Note that DCP profiles are not guaranteed to work identically across different raw processors.
2. Convert each of the DCP files to ICC profiles using dcp2icc (use the 32 bit binary, it’s easier), e.g.:
   $ dcp2icc 'Panasonic DMC-GX85 Camera L Monochrome.dcp' 5000
   (It’s not clear whether the colour temperature argument has any effect on the resulting output file; if in doubt and you believe your DCP profiles contain dual luminant values, generate several different output files using values of 2850, 5000 and 6500.)
3. Copy the resulting ICC files to ~/.config/darktable/color/in/
4. Install the lensfun-tools package and run lensfun-update-data to download the latest camera and lens correction data files.
5. If you have the GX80 variant, copy /usr/share/darktable/noiseprofiles.json to a separate folder in your home directory, edit it and change all occurrences of ‘GX85’ to ‘GX80’. Then modify your Darktable menu entry or startup alias to run darktable --noiseprofiles /path/to/modified/noiseprofiles.json instead.
   (This was a suggestion in a DT issue and I’m not entirely clear whether it’s necessary or whether DT knows they’re the same camera, but the current file definitely does not reference the GX80.)
6. Start Darktable and import your raw files in the usual way.
7. Select a raw to edit. Make sure you’re editing the .rw2 file and not the JPEG version if you create both.
8. Disable the basecurve module and set the demosaic module to AMaZE.
9. Go to the input color profile module. Your Panasonic style profiles should now show up in the profile dropdown. If you want to start by emulating one of these, try them out. I find the colour ones sometimes look a bit washed out initially (you may prefer to stick with standard color matrix default), although the Monochrome styles are a good starting point for B&W conversions.
10. Enable the lens correction module. If you’ve updated the Lensfun data correctly, this should have autodetected the correct camera and lens model.
11. Adjust the tonal range in the exposure module so that you have no clipping (automatic mode is often suitable).
12. Enable the filmic module (new in 2.6.0; if you don’t have this release, leave the basecurve and demosaic modules unchanged and use the traditional tone/colour modules). Use of this module still seems a bit voodoo but try the ‘auto tune source’ option for the tone-mapping parameters (might make things better or worse) and enable ‘preserve the chrominance’ for colour images (will probably need to back off the Saturation slider). Try tweaking the sliders from this point to adjust the contrast and shadow/highlight detail as required.
13. You might want to save a new Darktable style at this point for future use as an initial starting point.
14. Apply tone curve (e.g. S-curve to bring the contrast up a little), local contrast, sharpen, vignette, split tone, etc. modules as required. If you find the image a little noisy in smooth tone areas (e.g. blue sky), enable the raw denoise module with a very low noise threshold (e.g.0.002). If image colours still look off, try some of the other white balance presets.
15. Other tricks: to make the image pop a bit more, try the ‘boost’ preset in the highpass module or ‘clarity’ in the equalizer (but lower the mix value to 200-600 range). For a glow effect in B&W, apply the bloom or lowpass modules with a blend mode of multiply, and add masking as required. For landscapes, enable haze removal and try the graduated density module on bright skies.

One nit to watch out for with DT 2.6.0: if you use a non-standard window manager (such as fluxbox), you might encounter bug #12387. Should be fixed in 2.6.1.