On this page of our game guide to Batman: Arkham Knight you can find a description which explains how to find a way to infiltrate the Stagg Enterprises airships. Our walkthrough indicates how to defeat all opponents and where you should use the explosive gel.
Head towards Stagg Airships located right near Founder's Island. Your next destination is the watchtower. As you go there, watch out for the flying machines with guns (red cones). Avoid getting close to them to prevent being noticed.
When you reach the watchtower, you will notice that it is occupied by thugs supported by a drone. It is not your only problem, because there is also an enemy with a device that can reveal Batman's detective mode. You can avoid this problem in many ways. You can for example activate the detective mode only for a short period of time, several times (the enemy won't be able to detect your position). Also, you can resign from using this mode and rely on your own observations. If you choose to do so, it is good to move around when hanging from the edge of a building and hide behind covers when checking the area. It is obvious that the enemy with the Detective Mode Scanner should be the first person to deal with.
Besides the thug with the scanner, you should also watch out for the drone operator. He is located in the middle part of the tower. Good news is that he cannot move from his position. Lastly, deal with other thugs, but remember that they can suddenly change their positions when they notice that their allies are gone.
After securing the area, locate the terminal that controls the turrets guarding the airships. Spread Explosive Gel in this place and detonate the terminal.
Note - from now on, you can complete new side quests that consist in sabotaging other watchtowers. Of course, it is not required to complete the main story.