Your forests look pretty well already, you should first finish most of the underlying terrain before tweaking them more. The coasts are too smooth and need to be fractalized a bit. Searching this forum should turn up some techniques you can apply in GIMP.

Mountains are otherwise good but for the clean border between the snow caps and the bare rocky slopes. You can fix this by adding some blur (and Filters → Noise → Spread, in case you have the snow caps on their own layer with a mask)