Look in the "js/worlds/extra" folder to see what are available. Exclude the ".js" from the above.
This page will help you generate a shareable URL that will automatically load a world and/or robot configurations json file.
The json files MUST be hosted on a server that supports Cross-Origin Resource Sharing (CORS). Most hosting services DO NOT support this.
The only free service I know that supports this is Github. You'll need to create a Github repository, upload your file there, then either obtain the link to the raw file or Github page (...see this page for Github pages setup).
Alternatively, you can host the json files on your own server and configure it to allows CORS. See this page to learn how. Your webserver must support https.
If you are hosting Gears on your own server, change the first part of the generated URL accordingly. If the json files are on the same site as your hosted Gears, you can safely ignore the CORS error. You can also ignore the https error if you are hosting Gears on a http site.