Following guide contains spoilers so reader discretion is advised. Furthermore, before you try this trick to farm Green Gel, it is advised to upgrade your pistol and harpoon bolts.
In order to do this you must first select chapter 7 of The Evil Within on survival difficulty. While going through the level, make sure to conserve as much ammo as you can, especially the trap parts you collect.
Most of the enemies on this chapter can be killed using the built in spike traps. Doing this will allow you to reach the Keeper boss fight with around 30 traps and a healthy amount of ammo for your different weapons.
Now the farming kicks off. Once you have reached the boss, proceed as normal and shut down the poison gas. After closing the final valve, players will have the option to leave and finish the chapter but instead of leaving, remain in the area and keep fighting.
The Keeper boss will spawn from the safes nearly 30 times and each time you put him down for good. He will leave behind 2000 Green Gel.
As you have already fully upgraded damage on harpoon bolts and pistol, it will take one harpoon bolt followed by one pistol shot to kill him.
Once he respawns, repeat the process and by the end of it; you should have around 70,000 Green Gel at your disposal. Keep in mind that the Keeper has a 2 second spawn shield so don’t waste ammo during that time.
If you are running low on traps try to replenish them from the Keepers barbed wire mines. Wait for the Keeper to diploy his mines and kill him.
After that use the time he takes to respawn and disarm the mines. It is advised to have a sniper rifle or a shotgun in case you run out of harpoon bolts.