The only XML files that Gimp can use are SVG files (because these are image data: coordinates of points, etc…).
The XMP file from Darktable is just the description of processing steps applied to the raw file and only Darktable would know what to do with it (and, of course, the initial RAW file). If you want to further edit with Gimp an image that you worked on with Darktable, you have to export it from Darktable using a format that both Darktable and Gimp understand (JPEG, TIFF…).
You can also install Darktable as a Gimp plug-in. Then, when you open a raw file (CR2, NEF, DNG…) with Gimp, it fires up Darktable so that you can do the conversion, and when you are done with Darktable the result is loaded in Gimp.