how to get an arc of cello's gate

I think you mean the Blue Gate Cellar Key in ARC Raiders. You can get the “arc” by looting the key from random drops, with higher chances in residential areas, and then use it on the Blue Gate map; one guide places the cellar south of Olive Grove, up a small hill past the tree, and notes the key is single-use.

How to get it

• Loot keys from containers and general drops; they’re random, but residential areas have a better chance of giving keys.

• Once you find the Blue Gate Cellar Key, keep it safe because it burns on use.

• Bring it into the Blue Gate map and head toward the cellar location.

Where to use it

• One documented cellar is south of Olive Grove on the Blue Gate map.

• From the main road, go up the hill a bit, pass the tree, and you should see the door.

• The cellar opens with the Blue Gate Cellar Key and contains randomized loot.

Practical tip

• Put the key in a safe pocket if you have one, so you do not lose it before reaching the door.

TL;DR: Farm residential loot areas for the key, then use it on the Blue Gate map at the cellar south of Olive Grove.