I would contact GoPro and send them feedback. It seems like they decided not to update the app, but they could change their mind if they get enough requests.
When Apple Watch first came out with watchOS 1, most apps ran on the iPhone but displayed on the Apple Watch.
When Cellular Series 3 came out, Apple announced that apps would need to run natively on the Apple Watch even when the phone was off or out of range. "Please note that starting April 1, 2018, updates to watchOS 1 apps will no longer be accepted."
This makes sense since the apps should work on the watch without needing the iPhone, especially for cellular watches. The problem is that Series 3 and Series 4 have made the Apple Watch more popular, but a lot of developers decided not to spend time and money updating their apps because they just weren't being used as often on the older watches.
Some apps are being added back as native Apple Watch apps, but the developers need to get enough interest from customers to make it worth the time and effort to rewrite their watchOS 1 apps.