You can use the Grub2 password for your root login.
Starting with Unified Access Gateway 3.1, the Grub2 edit password will be set by default.
The username is root and the password is the same as the root password which you configured while deploying Unified Access Gateway. This password will never be reset unless you explicitly reset it by logging in to the machine.
Note: Manually changing the root password by logging into the machine using any commands will not reset the Grub2 password. They are mutually exclusive. Only during deployment will the same password be set for both (with UAG 3.1 version and later).