Reality is like a coin; flip it and it may come up One; flip it again, and it might come up Many. There is one Reality manifest as One and Many, and although we who dwell in the realm of 'reason' can only see this is as absurd and self-contradictory, it is not truly so.
Guo Xiang's flip of the coin comes down on the side of the Many and his philosophy follows this to its extreme. But though I say "extreme", this is not intended to suggest that he was 'wrong' — a great deal of what he has to say, even at the extreme (or perhaps because he reached the extreme), is very insightful and helpful to understanding and living the Daoist vision. Where he might have done better would have been to also flip the coin over and explore that extreme. Nevertheless, as we see in politics, the extremes often meet.
Central to Guo's philosophy is the concept of self-so (ziran), an application of the principle of spontaneity (ziran) to the ontological roots of all things. Things spontaneously arise. The Universe spontaneously arises. The Universe is a collection of these spontaneously arising things. They have no exterior "cause" or purpose. Dao is not the "Source"; it is (as I understand it) this arising.
Because all things are self-so, utterly independent in their existence, their most authentic expression is to "align true to themselves" rather than to an exterior standard. The 'rightness' of a thing is doing what it is. Similarly, in relation to other things, 'rightness' is expressed in allowing each thing to be itself. Live and let live. Go along with whatever you encounter. This is what is meant by "chariot(ing) upon what is true both to heaven and to earth" and "rid(ing) atop the back-and-forth [disputations] of the six atmospheric breaths", which allows for "free and unfettered wandering". (Zhuangzi, 1)
This is not mere acquiescence to events, but making the greatest possible use of them; it is flying free upon their backs, not relying on one event (wind), as did Liezi, but using every event so as to be dependent on none. Nor is it intended as a blanket affirmation of every perverse failure to affirm the self-so of each thing; those who gratuitously harm others have not aligned true to themselves.
Guo believes he arrives at this understanding of self-so through a particular understanding of what is Dao. Dao is Non-Being. Non-Being cannot create Being. Therefore, nothing is created. Whatever exists is thus self-created and without cause. What this amounts to is an attempt to explain by way of metaphysics what Daoism states without elaboration: that Dao does nothing, yet nothing is left undone. Yet, Daoism, at least in spirit, does not attribute attributes to Dao; the concept of Non-Being is as meaningless as any other when it comes to the Nameless.
What Guo provides us are wonderful insights into the implications of Zhuangzi's philosophy. He essentially takes Zhuangzi's 'conclusions' and attempts to rework the 'reasoning' by which they emerge into a more traditional philosophy of explanations, which he then applies to life. To this, Zhuangzi might respond, "Yes, you're catching on; now it's time for another flip of the coin."
You can check out Scott's writings on Zhuangzi here.