The Group ID (GID) of a SimCity 4 Object is the second set of eight hexadecimal alphanumeric characters making up the object's Type Group Instance (TGI) address. As an example, in the TGI "AAAAAAAA BBBBBBBB CCCCCCCC", the sequence "BBBBBBBB" is the object's GID. Group IDs are used in SC4 to organize related objects of the same file type. For example, all Transit Textures have a GID of 1ABE787D.
A list of all Groups, and the format of the Instance ID's that go with them is here. Note that certain custom content uses a Group ID that is unique to the computer that generated the item, as part of a method to avoid conflicts in custom content.