On this page of our game guide to Batman: Arkham Knight you will find out how to open the main ACE Chemicals gate. Batman has to defeat a small group od weak bandits.
Head towards Martin Drescher, who is being held near the main gat to the factory. The room is occupied by several opponents. The best strategy is to land on the roof and then destroy the glass part so that you can ambush the thugs. Deal with all the enemies, which shouldn't cause you much trouble. After the fight, you will learn that Drescher was murdered earlier. Listen to a conversation with the Oracle and pull the lever to lower the gate.