If you haven’t already found it, a good place to start is the Web Thing API specification, which documents the API web things are expected to expose.
A web thing should act as a web server and have a URL which resolves to a Web Thing Description which describes its properties, actions and events. Currently in the gateway you can add a web thing by manually entering the URL of its Thing Description.
You might want to start by writing a parser which can parse a Web Thing Description from its JSON format and a user interface where you can manually enter a web thing URL. Then you could work on generating a UI to interact with a web things properties, actions and events via the Web Thing REST API.
Later you could start to look into listening for mDNS broadcasts (a way of discovering a web thing URL on a local network automatically).
Hope this helps