Get a different keyboard.
Depending on your needs, there are two that I like a lot.
Simple Keyboard
(no frills. just a keyboard)
Hacker's Keyboard
(pretty plain, but lots of options to customize)
If you want something with more bells and whistles, try SwiftKey.
Also, if I remember right (it's been a long time since I used G-Board), I think there is a setting in the app to remove the emoji key.
I might be wrong on that, or have been dealing with a different version of G-Board.