Here is the full list of all 20 GridMath achievements.
Complete 100 levels
Complete 95 levels
Complete 90 levels
Complete 85 levels
Complete 80 levels
Complete 75 levels
Complete 70 levels
Complete 65 levels
Complete 60 levels
Complete 55 levels
Complete 50 levels
Complete 45 levels
Complete 40 levels
Complete 35 levels
Complete 30 levels
Complete 25 levels
Complete 20 levels
Complete 15 levels
Complete 10 levels
Complete 5 levels