Hi! I was also searching for a function to control the Webthings Gateway with Apples HomeKit and found this post.
As an alternative to control the Gateway with an Apple Device running iOS, you can use the API of Webthings and the Shortcut function of iOS.
With Siri you can trigger a created shortcut. In the shortcut itself you are sending a PUT command to the API of the gateway to change the status of your smart Home device.
Attached you can find my created example. It is working very good.
