The new GCO is not compatible with the K7 inks and will not produce the results you are looking for. But, you can upgrade your printer to one of these Piezography Pro ink sets: Piezography Pro Glossy Only for 8 ink printers
Or you can switch black ink cartridges between matte and photo blacks and use the matte / glossy Pro ink set
Piezography Pro Matte / Glossy for 8 ink printers
The Pro system prints glossy in one printing rather than two printings as does the K7 systems.
The new Piezography Pro blacks are much darker than the K7 blacks.