I have been tinkering with a solution to access openHAB from Garmin devices, via the Webhook HTTP binding. Unfortunately the standard REST API of openHAB is effectively incompatible with any Garmin software, since the Garmin Connect IQ SDK does not allow the type of POST request required to send commands to items.
However with the Webhook binding it is possible to provide a simple HTTP GET interface and control openHAB via various Garmin home control apps that support such calls.
I have described the solution in detail on my blog: