That is an edge case that Google could but does not seemingly accommodate, but very well could. It’s likely not just that you have no GPS, it’s also likely that you have no mobile or Wi-Fi signal reception, both of which could inform on your location, if implemented.

That being said, why do you need your navigation to start while in your garage? Just start it before you enter your garage or right as you drive out of it if you want to benefit from GMaps benefits.