Lg v20 booted up in safe mode and changed gboard by itself. Rebooted: phone out of safe mode. Gboard still gone. Uninstalled gboard and reinstalled. Keyboard not changed.
Is GBoard enabled on your input methods? Did you try to change the input method (keyboard icon on the navigation bar) when typing text? Safe mode disables 3rd party apps from launching, so it might have changed your input method to stock because GBoard was disabled.