JCage, The darkening of the image when you turn towards the sun is a function of the automatic exposure setting in your GoPro and these filters will not change that. Basically what these filters will get you are:
1. Help eliminate the jello effect seen in your footage which is caused by a combination of vibration and high shutter speed chosen by the automatic settings in your camera to give you proper exposure for a bright scene.
2. Give you a more "filmic" look by getting you closer to a 180º shutter (or double the shutter speed of your framerate). ie, if you are shooting at 60 fps on your GoPro, to get the proper "filmic" look, you would want to shoot at 120/sec shutter speed to hit the 180º rule. Since we can't set this on the GoPro manually, we add ND filters to help the auto settings on the camera choose a slower shutter speed.
3. If you choose a ND/Polarizer combo filter, you'll also get the benefits of having the glare reduced and some help with the colors.
That's why the 3 and 6 pack exists, to cover you in more situations. You'll need to experiment with the settings on your GoPro (setting the max ISO down to 400-800) while adding one of these filters for more professional looking footage. I own the Polar Pro 6 pack.