You can write a Rule Machine rule that checks to see if a motion sensor is active.
But as far as speaking on the Google devices, I'm going to give it to you straight and people can scream and cry all they want, but it's the truth. Google's ability to do that without extra software, sucks!
If you setup a separate server that runs CASTWEB API, then yes, you can with decent reliability, get it to speak messages to you on a specific Google device, in the official Google assistant voice. Or if you install Google Assistant Relay on a separate server, you can get a broadcast message on all of your Google devices at once, but you cannot send the message to only a single device. If Google would stop being such jerks and just let you trigger a Google Assistant routine with at least a contact sensor or a motion sensor, then you wouldn't need a separate server to do this semi-reliably.
Want to solve the problem for $25 USD? Buy an Echo Dot. Amazon Echo devices let you trigger an Alexa routine with contact sensor or a motion sensor. So with a few lines of simple code on your hub, you can turn on a switch, and Alexa will think a motion sensor triggered, which will activate the Alexa routine and it will say whatever you want it to say, on any of your Alexa devices. No extra server needed, and it works very consistently.
To automate this, do the following
Create a virtual switch that turns off after 1 second (you can set the auto-off in the virtual switch device details). This will be the trigger for your rule to test the conditions. When you ask google "Is someone in the garage?", it should turn on this switch.
[Garage Status] Changed
IF [Garage Status] ON AND
[Garage Motion Sensor] ACTIVE
Speak the message 'The Garage is Occupied' <- If using Alexa, you will instead turn on the switch here that makes it think a motion sensor has triggered
ELSE-IF [Garage Status] ON AND
[Garage Motion Sensor] INACTIVE
Speak the message 'The Garage is Clear' <- If using Alexa, you will instead turn on the other switch here that makes it think a motion sensor has triggered