Android L and its "Material Design" is definitely a hot topic. Some have already extracted the apps from a system dump and made them available for the general public. But if you're looking only for the redesigned Google Keyboard app, then this XDA version might be the one you should be trying out.
This version, however, comes in a form of a flashable zip file, so you'll need to use CWM Recovery or TWRP to do that. There are instructions for manually installing it should all else fail, but that does require root access since it involves writing into /system/app. You also have to install or delete any trace of the current stable version of Google Keyboard if you do have that installed.
Should you manage to get past those initial requirements and installation process, you will be greeted at first by a regular Google Keyboard. Going to the settings and choosing the "Material" scheme, however, will transport you to Google's design vision for the future. You will see a keyboard that looks totally flat and whole, with no delineation between keys at all. There are also monochromatic emoticons as well, when letters and numbers just aren't enough to express your thoughts.
Of course, this is just from a developer preview of Android L and will be subject to much change. It might be a bit too early to jump into the fray, but if you fancy yourself as a daring adventurer, do hit the link below to get the full links and instructions to install this app on your smartphone. Tested devices include Nexus 4 with Paranoid Android, Samsung Galaxy S5 with Phoenix ROM, LG G2 with stock Android, and the HTC M8. Theoretically, it should work with any Android 4.4 device, but there is, of course, no absolute guarantee.
[NOTE] Some might observe that the "Z" and "Y" keys here are swapped. This is because the source of the screenshots (Which says "Write message", for the curious) used a German keyboard layout, which uses the QWERTZ layout more popular in that region. Rest assured that on an English (UK or US) layout, it would use the more familiar QWERTY system.