Can I destroy a non Chaos Gear with decree of unity?

No — Decree of Unity can only kill a Chaos gear. Its text reads "Kill an enemy Chaos ([C]) unit or gear," so a non-Chaos gear (e.g., a Calm or Order gear) doesn't meet the targeting requirement and can't be chosen (Rule 355.6). Chaos gear, on the other hand, is a perfectly legal target — Gear are Killable per Rule 148.1.a.4.