in versions [1.5.4 b5831] (CORTEX v1.5.4 release notes) and later, 48 fps GoPro footage will fail to import.
The issue is only on the import, CORTEX will playback the footage once the clip has been created. So as a workaround, you can install [b5735] (CORTEX v1.5.4 release notes) side-by-side, import the footage in that version, then switch to the current version to continue the rest of your work.