To be very clear, gypsum board by itself is not a suitable or durable wall or ceiling material. It can crack in half if you lean on it the wrong way. If it's not supported at proper intervals, it will sag and crack over long time periods because it is not rigid. In fact, spanning large gaps and letting it sag is how gypsum board is purposefully bent by professionals to match smooth contours.

Gypsum board only gains strength because it acts as a system with the dimensional lumber backing that it is mechanically coupled to by screws. The tape and mud add a small amount of additional mechanical stability.