Hunting commander Gallagher
In order to start this mission, you have to get to location [962, 1402], specifically to the shiny circle on the ground.
New objective: Hunt and consume commander Gallagher
As you probably remember from the last mission, it is best to start the hunt with getting to the rooftop of one of the highest buildings in the area. Only then send an impulse (press left analog stick), so you'll easily find the location of the commander you're looking for (screen above).
The exact location of commander Gallagher is [725, 1425]. You do not have to specially disguise or avoid viral detector, because when you'll try to grab and consume him, Gallagher will automatically defend himself and be revealed as Evolved one.
Chasing commander Gallagher
New objective: Chase commander Gallagher
IGNORE Blackwatch soldiers stationing in the area and start chasing Gallagher. Notice the bar visible in the right upper screen corner. When you are far from the commander, the bar decreases. If it gets "empty", you'll fail the mission.
During the chase, help yourself of course with sprint, running over walls, long jumps and gliding. Do not try to grab or attack Gallagher too early, because the chase will automatically end after getting to the target area. You'll watch then a new cut-scene.
New objective: Tear off a rocket pod
After the start of a fight with hostile helicopter, the game will unlock a new attack. You'll be able to tear off a missile launcher mounted on the machine. Follow the instructions show on the screen - start with jumping towards the helicopter (A button).
Right after performing the jump, press the B button to grab the helicopter. The next step is to press X button (Weaponize), so Heller will focus on the launcher. Finally, same as with previous attacks on APCs, rhythmically press X button until you tear off the launcher.
New objective: Destroy Gallagher's helicopter
It is beast to use newly obtained missile launcher to destroy helicopter, and this is also a bonus mission objective. Target the helicopter (left trigger) and start shooting missiles (X button). The successful action will be rewarded with a very impressive cut-scene.
Defeating commander Gallagher
New objective: Defeat commander Gallagher to learn about the Whitelight program
The fight with Gallagher is very similar to the one with Dr. Koenig, which took place in the yellow zone. First of all, try to dodge your opponent's attacks, especially that Gallagher uses very powerful blades.
Look for an opportunity to perform quick attacks with claws or tendrils. From time to time Gallagher will be sticking his blades into the ground, becoming vulnerable for few moments. So you might use this to run towards him and press a button indicated by the game (X or Y).
You can try to attack Gallagher with weapons taken from local opponents or for example special, Devastator attack (screen above). Notice also, that the fight takes place in a public area, so you can easily regenerate by consuming civilians. Keep attacking Gallagher until his health bar is almost empty.
New objective: Consume commander Gallagher to learn about the Whitelight program
The only thing left is to approach Gallagher and consume him (B button). When you finish the fight, you'll be awarded with new ability - Blades. The game will automatically replace Claws with newly obtained Blades, but you can adjust everything precisely by opening "powers window" during the game (hold LB button).
Notice, that you can test right away the strong blades attack, titled Tornado, which is activated by holding X button. This is very good attack to eliminate groups of enemies standing close to each other.
New objective: Exit the alert
Depending on your preferences you can stay in this place, testing blades (screen above) or start fleeing, looking for some narrow alley where you can exit the alert. After ending this mission the game will unlock another one - Taking the Castle.