The GoPro's lens has a strong fish eye effect. This can easily be converted to a more 'natural' look with the free panorama stitching software Hugin
The horizon can be leveled by adjusting the projection in the 'preview panorama' window (icon in tool bar or <ctrl> + p). This action can best be performed between steps 5 and 6 in the previous section.
The final crop can be made with your preferred image editor. I like Gimp, the poor man's Photoshop.
For more examples: see this page