There are plenty of posts about GB but basically it is designed to check alertness when doing repetitive tasks.
Work your way through selecting any boxes with four dots. You are allowed to backtrack one space if you need to, you must do as many boxes as possible in the two minutes then change to next sheet and if you can make it to half way down the pages with minimum errors you are doing well. Practice really helps!
I hope this was what you wanted to know but let me know if there is anything else.