Right, GDSII contains layout information. This information should be transferred on wafer to create IC. This is called pattern transfer. Pattern is transferred on wafer by means of lithography. It may be photo lithography, e-beam lithography, X-ray lithography. Photo lithography is commercially used lithography. Cost of other lithography techniques is very high that make them difficult to use commercially. But they can provide better resolution than photo lithography , means they can create smaller features. For lithography we need to create masks. In case of photo lithography it is called photo mask. GDSII used to create masks that are used for pattern transferring.