How to enable Google Keyboard?
All disabled apps are listed on Settings &gt; Apps &gt; Disabled, where you can re-enable any of it.

If for some reasons or in certain situations, Disabled tab does not appear (like in my case) and restarting device does not help, then you can open menu in Apps (three dots in right top corner) and select Reset app preferences. 

After confirming all your disabled apps  become enabled again.