The GoPro 5 and 6 series have notoriously bad sound recording ability’s. This is very noticeable when using a gimbal as they pick up the motor whine. The 4 series is a lot better but the camera isn’t water proof without the case (gimbal usage). Using an external mic is the only answer with the 5 & 6 but you can’t use one with gimbal and you need to buy the £40 stereo analogue-to-digital converter too.
The alternative is to use an external sound recorder or in my case a second camera (sony as-30). Using a rode smartlav + and a TRRS to TRS adaptor the sound quality is vastly improved. It does require a bit more editing but that’s easy enough.