docs: Added installation instructions for gentoo (#556) [skip ci]

docs/getting-started/installation.md

@@ -105,11 +105,23 @@ yay -S overseer
 ```
 {% endtab %}
 
-{% endtabs %}
+{% tab title="Gentoo \(3rd Party\)" %}
+Portage overlay [GitHub Repository](https://github.com/chriscpritchard/overseerr-overlay)
 
+Efforts will be made to keep up to date with the latest releases, however, this cannot be guaranteed.
 
+To enable using eselect repository, run:
+```bash
+eselect repository add overseerr-overlay git https://github.com/chriscpritchard/overseerr-overlay.git
+```
 
+Once complete, you can just run:
+```bash
+emerge www-apps/overseerr
+```
+{% endtab %}
 
+{% endtabs %}
 
 ## Unraid