If you are looking to improve your Gopro Videos along with removing the Fisheye Distortion, then here is a fantastic solution. Peau Productions offers many different lens's that will do this. The one in my review is the 3.97mm and I am very happy I had this done.
Also with the Lens change you should consider their Filters. You will be amazed at how much they improve the quality of your videos. They are small, very light, made of glass and are of the highest quality.
Feel free to watch my review and 2 videos with the new lens.
Take Care, Grandpa Jake