The simplest way would be to create a standalone rule and use SHM Monitoring in the Classic app or create a custom automation in the new app.
However if you’re using the RBoy Apps LUM app, it has a built in door open monitoring function which you can use to monitor your garage door and have it notify you using a SMS, Push or spoken announcement.
Some other options:
You can check out include the Garage Door Management app: [RELEASE] Garage Door Open and Close Automatically when People Arrive/Leave with Timer and Actions which can also close the garage doors for you.
If you’re looking at repeat notifications with a custom messages and announcing over Alex/Bose/Sonos etc you can also check out: [RELEASE] Door Chimes and Notifications for Doorbells and Sensors