There are some basic facts I have learned by watching G-MAX tests.  I researched multiple sources on the internet, one of which was The Penn State Center for Sports Surface Research.

I learned on Penn States site that G-MAX is a measure of force and acceleration.  The purpose of testing a field is to determine how much force the sport surface absorbs, and how much the force from the impact the athlete absorbs.  A hard surface will absorb less force and therefore the athlete will absorb more, causing a higher probability of concussion.  When a field is evaluated, 10 different locations are tested throughout the field.  Penn State cites the maximum G-MAX for a field is 200 G, because over 200 G scores are related to life threatening head injuries.  However, the NFL follows a maximum score of 165 G.

There is no minimum score for a G-MAX, but a good point raised on Penn State’s site is the trade off for a field that is too soft, which would be a decrease in performance.  Their example is like playing soccer on a beach, as it would be too soft to perform well, and most likely tire the athletes out very quickly.