If you machine cannot handle gopro files convert the files to Cineform avi's, much easier to edit.
Also to set up an matching sequence do this:
Also check if the clips are interpreted correctly in the Project Window.
It's not that it can't handle it. It should handle it completely fine, It does with my 60fps and the 240fps wvga. Just seems to have some trouble on the 720p 120fps files. I would use cineform but I want to know, is there any loss in quality at all? Can I batch convert so that I can convert my files while I am out and edit when I am back? Does it keep the 120fps so I can work with slowmo later?