This is quite simply obtained by completing the description. Each game mode has a different Gold rating requirement, and you can achieve that rating by getting kills, assists, capturing points, etc. The better you play, the better your rating.
I found this easiest in Warpath as it requires 1,500 for a Gold rating and you're constantly in battle over the moving point, giving you plenty of attempts to gain points off captures and kills.
EDIT: Just as an fyi, this does not unlock until the end of the match, so don't fret if you notice it shows your Gold rating in the middle of a match but it didn't pop. :)