Wide: 14mmThe equivalency listed above is between GoPro's nomenclature and actual focal lengths of wide-angle lenses for 35mm film cameras or full frame DSLRs. A normal field of view (FOV), approximating that of a human eye, would be produced by a 50mm lens in such cameras.
What GoPro calls "narrow" is only so, relative to other GoPro FOVs; in conventional terms it would be considered a moderately wide-angle FOV, which would include 35mm film camera lenses up to 35mm in focal length.
The photo below (source) compares 14, 20, 28 and 50mm FOVs. Click photo to enlarge.