As far as I know, it's fully solid inside.
Simply make the garage door to open itself once you get close to it. I know it requires some custom mapping and a bit of scripting, that's the whole point of this suggestion topic. You CAN get inside from the air but it's not always a good thing, and it's a location that's not used at all imo.
There can be a few vehicle spawns as well so it won't feel empty.
It can be used by people to hang around as crews, hideouts, or simply evade the police.