GPR was developed by Oxford Learning Solutions to help the children who had already been failed by the classic approaches to reading tuition. Each child was guided individually towards success. So every child became a mini scientific trial because we knew that they were not progressing before and knew whether they had succeeded in catching up with their peers or not.
You will find that a surprising number of adults do sight read words, rather than decoding them. So it is not impossible to read in this way. However, people who do read in this way think it is normal to skip unfamiliar words, find reading names on maps almost impossible and have atrocious spelling. So it is not an optimal solution.
With a child you will tend to see progress in the early days, because the early reader books are often designed to be easy to sight read. However, as the books get more complex you will see more and more guessing until the child is on a reading plateau, with no further progress.
At Oxford Learning Solutions we look for 7 different potential causes of reading difficulty. Clearly the most important is the sight reading pattern I have just described. We call it Optilexia.
But there are 6 others too. Each of them can be a barrier to reading if not dealt with. This is a large subject and not directly related to GPR and so you should look at the 7 causes of reading difficulty if you want to know more.
Any GPR system needs to have good tools to spot the patterns of one of these complications and deliver the path to a solution either internally or through referral to an appropriate expert.