Welcome to the Centrify forums.
Please outline:
a) Version of Centrify DirectControl (adinfo -v) - only current versions are community supported (5.5.x)
b) 5.5.x is the version that is confirmed to work with Ubuntu 18.04.
That being said, please make sure:
a) That the GDM PAM configuration files contain the Centrify Stanzas (or include the system stanzas)
b) That you have restarted.
Note that things work well over SSH, so you may have to engage your GDM support lifeline for this.
R.P