Listen to a conversation with Gordon over the radio and head to ACE Chemicals factory located south-west from your current position. Your destination is quite far away, so it is advisable to use the Batmobile. After you get there, you will watch a cut-scene in which Arkham Knight will appear for the first time. When it's over, use the grapnel to get to the platform above the main gate. You will see another cut-scene and a conversation with Oracle will begin.