I want to know how to take shots like this on a motorcycle on a gopro hero 8. Is this effect done in post?
No need for post processing.
Relative to the camera, the motorcycle and rider are not moving (unless the rider waves the selfie-stick) so they are sharp. The background (clouds and landscape) are changing very little due to the distance, so they also look sharp. The road and nearby shoulder to move quickly relative to the camera, so are subject to motion blur, and this effect is more pronounced near the camera than far from it.
So, you just need the adequate shutter speed. Since the bike is moving at 90km/h (25m/s) and the blur tracks are roughly 50cm, the pictures are likely shot around 1/60s-1/100s.