I believe the only way you can do this is to embed the react app in a fullscreen iFrame. First you'd need to host the react app on a web host that supports it (replit maybe), and then you would stick this in the html.

<iframe src="https://react-app-url" style="position:fixed; top:0; left:0; bottom:0; right:0; width:100%; height:100%; border:none; margin:0; padding:0; overflow:hidden; z-index:999999;">
Iframes are not supported on your browser
</iframe>

I am sorry if this does not work (I have not tested it) but I use replit a lot and replit has great support for React so this is probably what I would do in your situation.

The following tutorial explains how to deploy a React app to a static GitHub Pages site: https://github.com/gitname/react-gh-pages

I would guess that it wouldn't work well if your app uses routing because reloading the page when the active path is not the root one would fail.