A game-based assessment is a method of assessing a candidates behaviours and traits, as well as ability, in a more fun and engaging way. They directly gather observable evidence of behaviours and traits that have previously been impossible to measure (in traditional numerical and verbal reasoning testing) – and most importantly, you have more fun doing them!
There isn't any way you can particularly prepare for the assessment, but just make sure you're in a suitable environment where you won't be interrupted!