This attribute indicates whether a specified DC is a GC server. Let TO be the object on which msDS-isGC is being read. If TO is not an nTDSDSA, computer, or server object, then TO.msDS-isGC is not present.
If TO is an nTDSDSA object:
If TO is a server object:
Let TN be the nTDSDSA object whose DN is "CN=NTDS Settings," prepended to the DN of TO. Apply the previous rule for the "TO is an nTDSDSA object" case, substituting TN for TO.
If TO is a computer object:
Let TS be the server object named by TO!serverReferenceBL. Apply the previous rule for the "TO is a server object" case, substituting TS for TO.