I've just received my copy of this today, and what a treat. For an average Pianist like myself (grade 6 classical a long time ago and basically a blues player now) the 136 pages here represent years of study. Some of it is frankly beyond me. I will never have the time learn the 20 pages of Rhapsody in Blue whilst I still have to work. This is a post grade 8 piece.
However some of the shorter pieces are more playable and the book contains "Three-Quarter Blues" apparently the easiest authentic original Gershwin piece. This is probably about grade 4 standard.
It has to be said though that these being authentic George Gershwin arrangements, the vast majority here are difficult (grade 7) or higher, so whilst you are purchasing some of the finest 20th century music, be aware that you will need to be a good reader or prepared to practice a lot!