Like SwiftKey's floating keyboard, you can move Gboard around after undocking it by using the small handle at the bottom. You can also resize it if you wish. And if you leave it untouched it for a bit, it turns transparent to show you more of the app behind it. The feature seems like a fantastic addition if you type with one hand sometimes and can't comfortably reach the letters on the opposite side. If you you can't find the option yet, it might still be in the middle of rollout and will make its way to your Android device in the near future.