Hi, Tonquani. Firstly, welcome to the Guild. It's nice to see a Scottish island being mapped!
A couple of thoughts...
I do agree that the green areas are a little flat and might benefit from some texture. One of the simplest ways to add to texture in Gimp is to use the Plasma filter. Go to Filter / Render / Clouds / Plasma and run the filter. Then go to Colours / Desaturate. Then set the Layer blend to Overlay and adjust opacity if you like. I use that filter fairly often, and it gives a nice effect with very little effort. Alternatively, try a big grunge brush on low opacity and start painting!
I think my biggest nitpick on the map is with the white outline around your labels, particularly the text at the top of the map. The outline isn't too bad on the green areas, but it doesn't work quite so well against the blue sea background. The paragraph of text is much harder to read than it should be.
Another thing you might consider for future maps is to add a legend to show what are roads and paths etc. (I'm assuming the grey lines here are paths?) A small legend at the bottom left would balance things nicely.
Anyway, just some thoughts for your consideration. Looking forward to seeing your next map. Maybe a bigger Scottish island!