Well, something strange happened in the update to GIMP 3.0.4 - it's likely a bug.
Previously in GIMP 2.x the paper size problem was the same. It has just never worked and AFAIK this has also been widely acknowledged in the GIMP community. To illustrate, this is the print preview I get after selecting 30x45cm custom paper size in the print dialog on my Epson 3880, landscape orientation with 40mm margins left and right:
The preview is for something like A6 paper size in portrait orientation. There's no way to fix this, apparently. It just shows how there's no people with an active interesting in printing on the GIMP core development team. It's a blind spot.
There's one more finesse to this - if GIMP knows you have RawTherapee, it will act like Camera Raw does for Photoshop - you open a raw file in RawTherapee and when you are done making edits, it passes the open file over to GIMP where you can keep working on it.
Yeah, although that, too, is kind of cumbersome. If you go from RT to GIMP, RT will save a TIFF in a temporary folder which is then opened in GIMP. If you want to export/save the image after further editing, you have to leaf your way back to the original folder. Not a major issue, but it's one of the many little quirks that you have to tolerate in the GIMP/RT ecosystem. In that sense, Adobe still has the edge - but you pay for it. With GIMP, you get a heck of a lot more than you pay for, evidently!
Here's some more quirks on GIMP as it is now, at least on Windows:
* If you select multiple files in Explorer and choose 'open with GIMP' and GIMP is not already running, it may never finish loading the files you selected. It'll just hang.
* If you use one of the new (since 3.0) adjustment filters and then resample the image to a smaller size followed by an undo action, the filter layer may end up being masked to the smaller size the image temporarily was.
* If you apply a dynamic adjustment on a selection, there seems to be no way to edit the automatically created mask. If you want to modify the mask, you have to discard the filter layer, make a new selection and re-do the filter.
* There are severe performance issues that pop up once GIMP has been open for a while. Simple actions like resample actions that usually take a second suddenly start taking a minute or more; same with curves etc. It's unpredictable if and when this happens - just that it'll happen sooner or later, but always within a matter of hours. It appears to be some kind of memory leak.
* If you scan a big image directly into GIMP, the TWAIN plugin will almost always crash. The image will often be transferred, though. This is also a 'feature' that's been with us for years, that's recognized, but for which there's just no active willingness to fix it.
*...and many, many more.
None of this is necessarily a dealbreaker and as said, GIMP is insanely good value as it is (hey, it's free), but it's nowhere near as robust or streamlined as Photoshop.
I haven't used Darktable, but much of what I said about RawTherapee would apply to it as well.
Last time I tried them both side by side, DarkTable was pretty much equivalent to RawTherapee, but DT suffered from some minor quirks especially when working on files on network shares. That made me lean towards RT.