Well the grades are the auction grades or rating. When a car is auctioned in japan it is given a grade out of five according to is condition. eg 4.5/5 means that the car is as good as new and it is virtually the highest grade that a redcond. car can get.
And for X and G which are for japanese corolla are just like XLI or GLi in pakistan. Corolla X is the standard model like XLi (i am not sure but maybe Xli has been derived from X) and corolla G is a better model with wooden texture etc.
https://www.pakwheels.com/forums/t/what-are-car-grades/45861