I use raw therapee not dark table and also run Linux. Behaviour should be very similar. When I open an ex RT image in the GIMP it detects the profile RT uses in images and asks if keep or change to standard sRGB. I’d expect GIMP to behave the same way on all platforms if the image profile is not a standard sRGB. Essentially it is an sRGB package. RT happens to use an equivalent. Not sure why as the sRGB profile is not copyright protected and is free to use. So behaviour there should depend on what DT uses.
On Linux the GIMP will pick up and use a monitor profile automatically. It can also be selected via edit-preferences-colour management. Rendering intent can also be set their. You may find it defaults to absolute for display as there are no changes in output media and perceptual for softproofing. That can also be changed to the usual variations if needed.
There is an opensource display calibration utility called DisplayCal. It’s compatible with the majority of colorimeters and spectrometers. I’m not keen on display a series of images and adjust etc but a better set than most is on this site. One in particular should help you set brightness correctly or at least balance that up as well as it can be. That’s the one with all of the split squares maybe along with the first black point test.
The spiit square test is pretty extreme. What I have found is that monitors often don’t show a number of the splits at the dark end or the bright end. The black test is likely to show faintly at 5 or so but you may have to accept a number higher than that.
Some find DisplayCal difficult to use. It’s best to stick with it’s default settings initially. The important stage is the first one where on a normal monitor the brightness and colour channels are adjusted to get as close to 120 cd/m^2 and a colour temperature of 6500K. The important aspect is the error it shows = de. It needs to be as low as possible and that may mean departures from the ideal numbers. Give or take 100k isn’t going to make any significant difference really. Any contrast setting needs to be set to factory default. Personally I favour ColorMonki colorimeters. Some use Spiders. Even a used on is probably a better bet than not having one at all. Mac and Windows users can just stick with the software that comes with colorimeters
Not sure about laptops etc but all monitors are supplied way too bright these days as it makes the dynamic range numbers better. Most have adjustments going 0-100 as well rather than the old 0-255 which means they wont set so precisely.
One more thing. It’s not that unusual for Nikon users to stick with their raw development package initially and then export to another editor. The reason for this is down to how Nikon generate camera profiles so profiling the camera makes sense. Maybe I should put maybe after that.