Re: Good luck.
Users are free to post what they want in the knowledge it will be published, so this wouldn't be relevant to GDPR even if it was in the EU, which isn't. The app itself is not that different in principle, let's say, from a weather tracking app. It wouldn't surprise me if there weren't a myriad of similar apps for colds, etc.
In the US the problems would arise if the app were making claims based on the data submitted. But even then it would be the FDA who decides. Letting the various app stores act as gatekeepers is a problem.