I have done a texture in GIMP and imported it in Unity but for some reason the texture appears to be squashed in the sides.
Image done in gimp:
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If you want to use it as sprite then change the import settings 'Texture Type' to 'Sprite(2D and UI)'. This will bring it back to its original dimensions. If you want to use it as texture then this wont matter, the texture depends upon the 3d object UV mapping, so it will scale properly.