GMS 5 and GMS 6 - just like VSys One prior to version 2.1 - use only Compatibility Mode security, regardless of the mode selected in VSys One. This is because Advanced Mode security was not defined when these programs were created, and as such they don't know about any of the new security features. This means that in general if you're using these programs on the same database that VSys One is running then you'll want to use Compatibility Mode within VSys.
It's possible to mix-and-match security modes. Enabling Advanced Security in VSys One has VSys One using that mode while GMS and older versions of VSys use Compatibility Mode.
What fields/settings are held in common between the two modes?
Everything else is different. If you want to use Advanced Security in VSys One while sharing a common database with GMS 5, for example, you can use VSys One to define user accounts and those accounts will exist and be valid in both systems. Assigning security rights, though, needs to be done separately in both systems. This means that you'd use GMS 5 to give each user rights within GMS, and use VSys to give them rights in VSys. Changes to a user's rights made by one system, VSys for example, would not be reflected in GMS.