Pay heed to
@Steven Lee 's comments.
Adobe's software reigns supreme in terms of effectiveness, usability and productiveness. But you pay for it, and I refuse to buy into their
extortion licensing scheme because I can't justify it for my hobby use. Had I been professionally engaged in image-processing, I'd just pay up, because then it would be worth it IMO.
GIMP & Darktable or RawTherapee (I prefer the latter) works alright. Frankly, I don't see the need for either Darktable or RawTherapee unless digital camera raw files are involved. Given a decently transparent folder structure with sensible names (e.g. date followed by subject/keywords), a bunch of png's/tiff's/jpg's in folders has always worked fine for me by means of an 'indexing' system.
GIMP has come a long way in terms of usability and functionality. Especially recent UI improvements are meaningful IMO. Subjectively I'd say they're roughly at the level where Adobe was around Photoshop 7 in terms of effective productivity (features + usability/accessibility). This puts them 'only' about two decades behind Adobe. But keep in mind PS7 was already pretty usable as a photo editing tool. It's not a bad performance for something that's free to use.
The main problem with GIMP is the lack of dynamic adjustment layers. With GIMP, you're limited to making duplicates of layers and then using whatever adjustments you want in combination with masking. This removes the possibility to 'adjust the adjustments', which is a huge benefit of the Adobe approach. In terms of performance (speed, memory use), Adobe also has a fundamentally better approach IMO.
All considered, I'm quite happy with GIMP as it is, especially given the cost. But it takes some getting used to if you come from Adobe, and there's some things that Adobe will just always be better at.
RawTherapee I use only occasionally since I don't shoot all that much digital, but for me, it gets the job done and it's more packed with features than I really need. For critical digital photographers, I would suggest carefully evaluating image quality performance against e.g. CaptureOne since there are very real differences also in this area between raw converters.