This achievement is earned by reaching level 5 as your ranking status.
For this achievement much for XP you're able to take advantage of any gametypes for earning XP towards your level 5 ranking.
-Campaign with up to 4 friends
-Online versus, or even private versus
-Playing Horde/Beast Mode will also earn you XP toward level 5.
UPDATE: As of November 18th 2011 EPIC has granted anyone who has/hasn't started Gears of War 3 an automatic 29/30 Event Matches played. And with XP being earned at 5, 10, and 20 events you'll already have earned XP to reach level 11 from starting the game. And once you've started a match the game will register you for your 30th Event match. This will you the following achievements:
-And once the match has been completed-
Award Winning Tactics (Earn an Onyx Medal)
Socialite (30 Events played, Onyx Medal)
However for any of these methods you'll be able to reach level 5 and earn this 5G achievement!
The following video is of my first Team Deathmatch game where the 29/30 Events played update is displayed.