Can i use Decree of Unity on a gear that is not chaos?

No — Decree of Unity only reads "Kill an enemy Chaos ([C]) unit or gear," so the target must both be an enemy's and have the Chaos domain on its own typeline. A gear that has no Chaos symbol (or one whose domains don't include Chaos) is not a legal target, even if it's attached to a Chaos unit — attachment doesn't pass domain to gear.

Note the two separate restrictions: it must be an enemy gear, so you can't kill your own Chaos gear with it either (FAQ 12661). If the gear is Chaos but you control it, it's still not legal.

If you meant a gear whose domain you're unsure of, check the symbols in the lower-right corner per Rule 134.1 — that's the only thing that determines legality here.