Two options come to mind.

1).
Darktable is showing you a representation of the RAW data, processed through it's own 'pipeline' - a series of computations that converts that data into something you can see on screen. One of the early stages is de-mosaicing where the sensors data is converted into RGB values for each pixel. This can be complex and slow so Darktable allows the user to set the quality of display for different sizes of display.

eg.

8.1.2. Lighttable
don't use embedded preview JPEG but half-size raw
Check this option to not use the embedded JPEG from the raw file but process the raw data. This is slower. (default off).
high quality thumb processing from size
If the thumbnails size is greater than this value, it will be processed using the full quality rendering path which is better but slower (default 720p).

8.2.1. Quality
demosaicing for zoomed out darkroom mode
Interpolation when not viewing 1:1 in darkroom mode: “always bilinear (fast)” is fastest, but not as sharp. “at most ppg (reasonable)” is using ppg + interpolation modes specified below, “full (possibly slow)” will use exactly the settings for full-size export (default “at most ppg (reasonable)”).

2). Darktable has lots of settings for colour management and applying icc profiles. The Colour Management System (CMS) uses the profile to make the display of the RGB values appear the same across different pieces of software and device. For Gimp and Darktable to display the image the same they must both be using the same icc profile and display intent. Both software's try to use the system display settings but it may be worthwhile checking what the setup is.

see 3.2.6. Color management in the Darktable manual.

and Chapter 11. Color Management with GIMP and 1.3. Color Management in the GIMP manual