Hmmm, would it have been better to find something that fits your needs before you ordered?
That said, I suggest that you try it before you begin cutting. That deep crescent butt is made to shoot off of the bicep, not the shoulder, so give an honest effort to try it out, as is, before you begin to cut.
IMHO, since the GPR has a LOP of about 14 inches, there probably is enough wood to trim the butt if you can be satisfied with a much shorter length of pull. That is up to you.
Another thought is that the more material that is removed from the butt, the less drop the stock will have. So, the question is, can you put up with a 13 something length of pull, and reduced drop, which will make it harder to align the sights.
My suggestion is to cancel that order and find something that better fits your needs.