Keyboard Android Lollipop has been leaked on internet since quite long time. There is two ways to install the Google Keyboard, with or without Root. At first, people just know that its application should be installed with root. But now, thanks to a developer who wants to develop the way to install without root, you now can enjoy it.
So, you can try it without root access needed. You also do not need to delete or uninstall Google Keyboard which has been installed beforehand. You just need to download the APK file and install it as usual.
You might find that the display is not changed much. But, this newest application adds the options for Material Dark and Light. But there is a note from the developer that there is one problem unsolved yet, when the keyboard is being used with landscape screen mode, there is crash happening. So, it is better not to use landscape screen mode in order to avoid any crash.