Jamie,
I have configured opengarage to integrate with OpenHAB using a Mosquitto Broker. I installed this using the options within the Openhabian configuration script.
Configure Opengarage to use this MQTT broker (in the opengarage options menu)
Install the MQTT binding in OpenHAB. (see binding details at https://docs.openhab.org/addons/bindings/mqtt1/readme.html),
I use the following garage door item in my items file
Number Door_Garage_Right_State "Garage (R) is[MAP(access_garage.map):%s]" <garagedoor> (gGarage) {mqtt="<[xxxopenhab:My OpenGarage/OUT/CHANGE:state:default]"}
and I have a map file to transpose the opendoor outputs to be consistent with the rest of my setup
I was new to MQTT and found an excellent resource at https://www.hivemq.com/blog/mqtt-essentials/page/2/
Hope this helps