The G2G6 badge is awarded when a member gets over 6000 points in the G2G. It's awarded manually, usually by me. I check about once a week and give out the badges when I see someone pass by 6000.
Sometimes though, someone sneaks past, especially if they post quite a bit at once and blow past the 6000 point mark.
If you have more than 6,000 G2G points and no G2G6 badge, just post here and I'll be sure you are awarded it.