You make it! It's pretty simple. Just type 'kit' into the search bar in your crafting inventory, and it will show you what you need. You can make the in the workbench or your inventory.

So it counts? Thank you. Cleared the game solo on Insane ~7 times, so it's more about the changes :3

Oh, by gather they mean to have it in your inventory.

Yep, either way counts. You can find them out in the world, get them as rewards or in air drops, or, you can make them! I usually take all the stuff I need to make a particular biome gear item with me when I go to do the challenges. It usually doesn't take very long and, if you've made a few 'smoothies', shots, whatever, it's done in no time at all... well, other than having to search for the special zombies in the desert and snow. Sometimes that can be annoying.

I see why nobody asked the question before - Have to move directly to the Wasteland (strongest area), because the building material cannot keep up. It's day 10 and the game is half way cleared.

Never mind, every biome requires this item ^^

Ugh, from my first Wasteland run it seems not worth it at all to move away from the forest. 400 bullets used for T2 infested (T1 monument), +100 bullets -20 gained of my gun type at 4 times the time needed of forest.
They just keep coming from the street while the zombies inside are way too tanky, robbing time and bullets.

Not to mention the 2 full days to move, 2 days to build a new base. While clearing the game (max gear) only requires up to 20 days.

2nd T2 infested: +85 -350 bullets (T6 pistol vs glow and eye zombies)

Hum, but I must say I find T5 armors.