It’s time for another Gray Zone Warfare double-take. Name your poison and spit it out. The poison involves taking a vial of poison to Fort Narith’s base to poison the garrison, but there is an alternative.
Name Your Poison // Spit Out the Poison | Gray Zone Warfare GUIDE | Mission Tutorial
Turncoat wants you to poison the soldiers because "they haven't been honest with him in the past." I don't see how that's our problem, but he offers a nice reward for delivering the killing blow. Lab Rat has heard of your efforts, however, and begs you to reconsider. You'll have to make a choice, but if you do, you won't be able to complete the other task.
Here's everything you need to know about the Name Your Poison and Spit Out The Poison tasks in Gray Zone Warfare.
You start the task by picking up a Poison Vial from Turncoat. He will send it to you via the message system. Once you have the Poison Vial added to your inventory, head to the Fort Narith Barracks at coordinates 141, 132. A water tank on a trailer bed is located just north of the most northwestern barracks building