How to log into Google Keyboard on Samsung phone?
Try this solution:

Open up your Settings
Navigate to Accounts
Click on Google
If you have multiple google accounts, find the account which you would like to use
Tap the account you would like to use
Now enable sync for the following things:

Sync App Data
Sync Google Keyboard (if you have the option to)


Reboot
Go to Google Keyboard settings
Sync, and have fun

Add a comment below if you have any questions, or require further assistance!