I have upgraded my ESX VMs from R81.10 to R81.20 and had the same warning both on SMS and GW !
Reason: See R81.20 (Titan) Release Notes: Software Changes
This section describes behavior changes from previous versions.
Gaia - The password for the Gaia GRUB (boot loader - maintenance mode) is a dedicated password (separated from the Expert mode password). You can configure the Gaia GRUB password during the Gaia First Time Configuration Wizard, or after the Gaia installation.
--> This is a new feature as the former expert pass also was the grub / maintenance mode PW...
CCSP - CCSE / CCTE / CTPS / CCME / CCSM Elite / SMB Specialist