What exact version of After Effects? What operating system?
Does the same footage play in Premiere Pro?
What are the details of the footage (codec, container, etc.)?
Did you install the GoPro app? If so, is it up-to-date? There was a bug that has been fixed. I am having no problems at all with any GoPro footage.
I Have AAE cs 5.5 and it does play in premier. Bur I primarily use AAE for my work. I have a Windows 7 ultimate 64-bit pc. the video is a .mp4 which should run fine. Codec unknown at the time