This may or may not be in the cards for you, but it seems you're setting up routing for a SPA.  Many client based routers offer **hash-based** routing precisely for this reason so that you don't have to configure your hosting provider to redirect urls to return index.

The landscape of solutions seems to be:

1. Use Hash-Based Routing

    
    
    <pre><code><b>USE</b>: https://example.com/index.html#inner/path
    <b>NOT</b>: https://example.com/inner/path</code></pre>

   * **React Router** - [`<HashRouter>`][7]
   * **Angular** - [`HashLocationStrategy`][8]
   * **Vue Router** - [`{mode: 'hash'}`][6]

2. Use the [custom 404 page][2] for gh-pages to [redirect all missed traffic back to index.html][3]
3. Choose a different [free static hosting provider][1] that allows basic url rewrites or redirects like:
  * [**netlify**][9]
  * [**surge.sh**][10]

It doesn't look like github supports this feature right now, so you'll either need to avoid deep link requests to the server or change to a hosting provider that allows you to do so.

**Further Reading**:

* [Can I create routes with react-router for a github-pages site?][4]
* [Add single page application support for Github pages][5]
* [S(GH)PA: The Single-Page App Hack For GitHub Pages][11]

[1]: https://codebushi.com/hosting-your-static-website/
[2]: https://help.github.com/articles/creating-a-custom-404-page-for-your-github-pages-site/
[3]: https://github.com/rafrex/spa-github-pages
[4]: https://stackoverflow.com/q/40776651/1366033
[5]: https://github.com/isaacs/github/issues/408
[6]: https://router.vuejs.org/guide/essentials/history-mode.html#example-server-configurations
[7]: https://reacttraining.com/react-router/web/api/HashRouter
[8]: https://angular.io/api/common/HashLocationStrategy
[9]: https://www.netlify.com/
[10]: https://surge.sh/
[11]: https://www.smashingmagazine.com/2016/08/sghpa-single-page-app-hack-github-pages/

I want to serve same index.html page for all (sub)urls in my app on gh-pages

is that possible?

right now when I go to repo/inner/path I get a 404

so I want to "force" github pages serve the same root index.html on all routes