Right now, only way to get gold clan tags is to be in a clan level 10 or higher on MW3. The tag must be typed exactly as it is on Elite. Characters don't work, so if your clan tag has characters you won't be able to type it the exact way in BO2.
Also, once your clan reaches level 10 on BO2 you will get the gold clan tag. No operations or challenges on BO2 yet so you can't level up your clan at the moment.