I am using Samsung S6 and I installed Google Keyboard on the device.
When I was using Google Keyboard on Nexus device, it was logged into my Google account. On Samsung, if I go to Settings -> Language & Input -> Google Keyboard -> Accounts & Privacy, the Change Accounts preference there shows No accounts selected and does nothing upon tapping.
Is there a way how to make it log into my Google account so that I can sync my dictionary and use other functions of Google Keyboard only available when logged in?