The built in integration only allows the use of switches, dimmers, thermostats, and bulbs. The terminology is all based on those devices. So, if your garage door is integrated as a switch, it would be on/off instead of open/close.
If you wanted more flexibility and a better integration for the garage door, you need to look into [Alpha] Community-maintained Google Home integration. That community integration will give you more flexibility and a broader scope of devices that can be brought into Google Home.
