The main problem is that the book suffers from some of Dick's worst writing. Surviving on the strength of his ideas, Dick was never the best actual writer, but here his writing approaches a level of hackery not often seen in his mature novels.
GPH was published in 1969, 15 years after his first novel, and it includes some whoppers like these:
He snapped the book shut. "It's talking about itsself." Coming over to his chair, Mali bent to read the text."
Um, how can she read the book when Joe already shut it in the previous line?
At street level he straddled a cracked and unrepaired sidewalk, took a deep angry breath, and then, via his personal legs, started north.
"Personal legs?” As opposed to what, specifically?
This novel is filled with stuff like this. Dick wrote fast and furious and apparently didn't have much time for rewriting or editing... he needed the money and his ideas flew out of him as fast as his fingers could type. What’s amazing is that despite all of this, the ideas that Dick presents are fascinating, especially for his time. Here's two examples:
1. A “Padre booth” in which, for the price of a dime, you can consult with the deity of your choice. Pages 47-8
2. The SSA machine, which reads the minds of two people and then presents a picture of their potential future together. Pages 63-4
The other thing Dick's writing does is present you with deeper philosophical musings above and beyond the plot. In GPH, Jim spends a moment at the airport questioning why he's going off to another planet to help the Glimmung - a minor deity - raise an old church from the sea. But he does so by thinking:
A man is an angel that has become deranged... Once they--all of them--had been genuine angels, and at that time they had had a choice between good and evil, so it was easy, easy being an angel. And then something happened. Something went wrong or broke down or failed. And they had become faced with the necessity of choosing not good or evil but the lesser of two evils, and so that had unhinged them and now each was a man.
To me, the best parts of Dick’s writing are often not plot-oriented - and certainly isn't his prose - it's the extra things Dick comes up with as he’s telling his story.