Check out Kinoveo.org for a free, open software solution. Just be careful. ALWAYS work from copies, not originals, and save to a new name. And watch out for filetype. It defaults to .mkv (Matroska) but can also do .MP4 and .AVI.
Its not a true video editor, with lots of bells and whistles, it is for motion analysis. It can do extremely slow motion, I think by interpolation, since I dont see any jerkiness, but if you set you GoPro up to do higher fps, I think it might do better. But it lets you write to file the slowed down version. Just watch the file size.