Hi Laura
Sorry it took you so long to receive an answer.
GDL Library Parts are govern by an inertial serial number "GUID" that is composed of 2 parts, the first part tells ArchiCAD what GDL subtype should it reference the new Library Part.
In simple english words for GDL novice: When ArchiCAD reads the object serial number, it will know what type of object it is (ex: door, window, furniture, etc), for more info read GDL guides and manuals
In your case, I guess you have created a new subtype from one that was already available with ArchiCAD, if that was true, then the new subtype that you have created will have a new GUID, but that GUID will reference it parent object, therefore nothing bad will happen.
A good friend of mine have once told me that I´m so brute that I´m capable of creating a GDL script capable of creating GDLs.