The issue you're having is with light metering. When you're looking out over the horizon, towards the sun, the camera is darkening the picture to compensate for the bright sunlight. This is then causing the ground to be underexposed.
It's generally better to underexpose than to overexpose as you can always brighten video in your editor and recapture some some detail. Overexposed footage cannot be recovered. Depending on the camera you are using and you're editor and committee grading skill, there are a couple ways to address this.
First, if keeping the ground well exposed is most important to you, then using the Hero6 Black is the easiest fix. Press on the back of the LCD until a large white box shrinks into a smaller one. The small box becomes the area the camera uses to meter light balancing. Place the box on the bottom of the screen and then press the check mark. Using this option will cause the sky to become brighter when the camera is metering for the darker ground, but it will keep the exposer bright enough to keep the ground well lit. If you want to keep the sky from getting blown out and too bright, do the opposite and put the box in the upper portion of the LCD. You want it to fall approximately to where the sun will be in the shot. Personally, I prefer the second option where the ground will be dark but the sky is not blown out. If you have a decent editor, you can brighten the image. If you use key frames, you can adjust the brightness according to where you are looking.
The second option can be done with either camera. Turn Protune on and use the following settings:
Shutter: Auto
EV Comp: -1
ISO Max: 100-200 (Hero6) 400 (Hero4)
WB: I prefer to set this manually but you can use Auto
Sharp: Medium or Low
Color: Flat
Your image isn't going to look great out of the camera, but if you use an editor like DaVinci Resolve (Best color grading editor), you can make it look great. If you don't already have DaVinci Resolve, get it. It's free and the best professional color grading software available.
