The msDS-isGC attribute exists on AD DS but not on AD LDS.
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:
If TO is a computer object: