To unlock this achievement, you will need to have burst a Gordo Slime and gotten a key.
Doing the above should net you the achievement below if that is your first Gordo Slime burst,
After getting the key, head back to your ranch and stand facing the exit to the main area.
Head north out of the area, take the northern slope up and continue on. In this area, you'll see a giant rock formation that is not part of the wall. Head behind the rock formation and continue on the path. Go down the slope and when you come to an intersection with a hole in the wall to the right and a path leading to purple grass on the left, head left. Continue and head up to upwards slope to your left. Head through the tunnel and go through the opening in the rock wall ahead. Head down the platform, go around the rock area in the middle and head right through the opening in wall to a bridge. Go over the bridge and get to the next area.
Be warned. There are feral Pink Phosphor Largo Slimes here. They will attack you similarly to the Tarr. Except you can tame them. Feed them food they like, so in this instance any type of fruit: Pogofruit, Cuberry, Mint Mango, Phase Lemon, or Prickle Pear. They will become tame and you can pass through without a problem.
Though if you like, you can just run.
Follow the path until you get to a bridge. Continue along the bridge and when you can turn left onto another bridge, do so.
Here you will see a large door with a keyhole. This is the entrance to the Moss Blanket. Use your key that you got from a Gordo by hitting
when in front of the door.
Your achievement will pop and if this is the first time you have used a key to unlock an area, so will this other achievement: