Do you want the motion detector to just notify you or to actually trigger an alarm if someone walks in the garage? If you just want it to notify you then you can use a motion detector but program it as a “no response” type of sensor with Alarm.com activity monitoring enabled on that sensor, and then set up a notification when the sensor is activated.
If you want to detect when the garage was left open and be able to close it remotely then the only solution is the Liftmaster MyQ garage door control. What model is your garage door opener?
If you are willing to give up the remote control than you can get away with just a garage tilt sensor. You would need to enable Alarm.com activity monitoring on it and then you can set up a “sensor left open” notification to alert you if you left the garage door open.