First gated area is located in the Dawn of the Dead endless zombie survival level of Operation 1: Unicorn Lair and it unlocks during the first few waves. There's a machine gunner located in this area.
To access the Dawn of The Dead level you will first need to unlock it. There's three different ways of doing so:
- Get seven bullets from the Operation 1: Unicorn Lair levels by playing through them in different difficulties. As there are seven total levels you will only need to play them on the easiest (1 bullet) difficulty which gives you one bullet per level. Playing on harder (two or three bullet) difficulties will naturally unlock Dawn of the Dead faster
- Collecting medals from Operation 1 levels and using medals to unlock the level
- You can also unlock all zombie levels from all four Operations by buying the Zombie Unlock special from the in-game store (2,99€)