Before starting I'm French and my English is not very good, so If you guys find spelling errors or incomprehensible passages you can let me know by writing a comment on the guide so that I can correct them. Thanks in advance.

In Gray Zone Warfare, the majority of high-tier loot can only be found in military gear crates or UN firearms boxes. The black appearance of these boxes makes them easily identifiable.
On the map, there are 10 military gear crates and 4 UN firearms boxes (according to my knowledge, some game updates may change the amounts).

In Fort Narith, there are five military gear crates that can be found. A key is required for three of them.

• In the first floor of the air control tower
  
  
  
  
  
  
• Just in front of the bunker, next to the armored door
  
  
  
  
  
  
• In the room A103 in the dormitory
  
  Key needed : FN A103
  
  
  
  
  
  
• In the strategy room on the second floor of headquartes
  
  Key needed : FN HQStrg
  (I don't have the key so I don't have a screenshot of the crate)
  
  
  
  
• In the garage office
  
  Key needed : FN GOfc
  (I don't have the key so I don't have a screenshot of the crate)
  
  

In Midnight Sapphire, there are three military gear crates that can be found. A key is required for two of them.

• In Elena Villa's garage.
  
  Key needed : MS ElenaGar
  
  
  
  
  
  
• in front of the hotel place
  
  
  
  
  
  
• In the golf club storage. The door is located outside the building.
  
  Key needed : Ms GCStor
  
  
  
  

In Tiger Bay, there are only two military gear crates that can be found. A key is required for one of them.

• Second floor. It's in the open, not in a shop.
  
  
  
  
  
  
• Tiger Bay Central Armory Room. In the back of the shop.
  
  Key needed : TBC Arm
  
  
  
  

In Ban Pa, it is possible to find only a UN firearms box. No key is required for it.

• In one of the stilt houses
  
  
  
  

There are a total of four crate scatser in the map. Three UN firearms box and one military gear crate. No key is required for it.

• 1 In a truck in a ambushed UNLRA convoy
  
  
  
  
  
  
• 2 In a LAF checkpoint in the forest close to the Ground Zero zone
  
  
  
  
  
  
• 3 In a truck in a ambushed UNLRA convoy
  
  
  
  
  
  
• 4 In the back of a red car in the crossroads behind a ambushed UNLRA convoy
  
  
  
  

Thank you very much to all of you for using my guide and I hope it helps you to get good stuff and a special thank to Vadaam__ who help me with this guide.
If you guys find spelling errors or incomprehensible passages you can let me know by writing a comment on the guide so that I can correct them.