It is worth noting that Christianity does, in fact, have its own branches which wouldn't care or even notice if God didn't exist. Things would move along pretty much as they always have if it turned out He never existed or if He just up and disappeared. Christianity is infected with what often gets termed 'moralistic therapeutic deism.' These churches have given up worship of God in exchange for moralizing their members - making them good citizens who obey the golden rule and teach their kids that God will let them into heaven if they just follow God's rules and check all the marks on the list.
Many churches have such little belief in the sovereignty of God that they believe they must be the ones to make God come within their midst. Inspired by Charles Finney and his axiomatic belief that it is man and not God who decides when revival happens, much of Christendom has set to work doing religion when the show has already left the building. The sad reality is, many churches have gotten so used to 'making church happen' that it now bears no urgent dependency upon the Divine. If God were to depart, such churches would scarcely even notice.
Other experiments in fulfilling John Gray's supposedly 'novel' idea have been tried. Back in the 60s, Thomas Altizer and William Hamilton posited a 'Christian' denial of God's existence known as the "God is dead" movement. It is of course, a relic of the old liberalism which is long forgotten by most Christians today. Such men truly argued that God is dead and gone - most specifically that Christian theism is not true. And yet they insisted on calling themselves Christian. They may have made great elders in most churches in the PCUSA.
In truth, attempts throughout recent church history have been made to separate the existence of God from the practice of religion. One need only look at the demise of the mainline protestant denominations in the United States to see that a divorce of God from religion is not sustainable in the long-term for more than the obvious reasons. When experience of God and devotion to God is split off from liturgy and sacraments, one is left with a rank and odious procession which is more a funeral for the divine than worship or celebration.
In the end, it is sad to say that despite my own protestations, I'm afraid John Gray's arguments may have been unwittingly adopted by large portions of American Christianity without their conscious awareness. Is there help for such churches? Sure. But the changes needed are fundamental, and I'm not holding my breath waiting for them to turn around.