Original: Knights and Dames Grand Cross use the post-nominal "GCMG"; Knights Commanders and Dames Commanders use "KCMG" and "DCMG" respectively; Companions use "CMG".
Simplified: Knights and Dames Grand Cross use the post-nominal "GCMG". Knights Commanders and Dames Commanders use "KCMG" and "DCMG" respectively.