The GTASC (Great TrueAchievement Score Challenge) is run every year by planting42.
How it works: There are two separate but concurrent contests - the Individual and Team Contest. This year, the Teams will consist of 3 gamers.
The contest runs for 48 periods of one week, and at the end of each period the lowest scoring teams in each contest are eliminated and the leaderboards are reset for the next period.
GTASC 2019 Scoring Change: This year, the contest leaderboard will be based on your TA Difference for the period. TA Difference is the difference between the TrueAchievement score and Gamerscore you have earned during that time - so getting harder achievements is now more rewarding that it has been previously!
Eliminations: You must try to keep yourself or your team above the elimination line at the end of the period. Those falling under the line at the end of the period go home empty-handed.
Important note: TrueAchievements only track achievements earned while you are connected to LIVE (aka online achievements). 'Hiding' - or appearing offline doesn't affect this as you are still connected to LIVE and receive timestamps for your achievements.
See the GTASC contest forum for more info.