A gimbal is unnecessary. You do need a quality smartphone such as the current iPhone 13 Pro. Its stabilization is as good as a gimbal and much more convenient. (I do not have any experience with Android phones, but others who do may want to chime in.) Of course, a quality smartphone is likely to be more expensive than a GoPro.
A quality smartphone mount is also required. I use the Peak Design mount. It holds my iPhone very securely with a combination magnetic and mechanical latch that allows very fast attachment and release of the phone in either vertical or horizontal orientation. Unfortunately, the Peak Design mount is also expensive, and it requires the Peak Design phone case.
I am a professional photographer and documentary filmmaker. I have shot with gimbals from the days of the Steadicam Jr. more than 30 years ago. I currently own a Zhiyun Smooth gimbal, but gradually stopped using it as iPhone and GoPro cameras became really good at stabilization. Just yesterday, I shot one of my daughter's landscape design projects with my iPhone 13 Pro (walking, not on a bike) and a DJI Mini 3 Pro drone (which, of course, has a gimbal). On my electric bikes I have used the GoPro Hero 3 Black (no stabilization), and the GoPro Hero 9 Black cameras in addition to the iPhone 13 Pro.
By the way, Court Rye, the owner of this website, appears to shoot his video reviews with a hand-held gimbal. You may want to ask him directly for his recommendations. However, his purpose is not so much to shoot the view as he rides as it is to shoot the bike he is riding.
One of the issues with using a gimbal is that your arm gets tired from holding it within a relatively short time. Something to think about if your bike rides are hours long. Plus, riding one-handed increases the chance of an accident.
