First: If you can't self-determine, you shouldn't call yourself an anarchist. Anarchy means "NO rulers"; not "no gods but humanity"; not "long live the CNT." It is individualistic at its core, not communitarian. If you're not self-determined, you're a product of someone else's perspective; you've defined yourself vicariously through an other, and that's the opposite of anarchy.
Now: I recommend reading Diana Judd's short review of Gray's book "Straw Dogs."
Here it is as a pdf: http://www.logosjournal.com/judd.pdf
A Political Science professor at Rutgers, Judd is smart enough to critique not only Gray's specific statements ("On the one hand," she writes, "Gray states that 'the origins of science are not in rational inquiry but in faith, magic and trickery,' while on the other hand he equates science with technology, from which, according to Gray, its power flows."). She also attacks the structure of the book itself ("...the reader is treated to an array of disconnected quotations from Aristotle to Zarathustra, none of which serve to illustrate a coherent argument.") I agree with her on both counts.
I personally dislike Gray's use of the Gaia Theory to frame his argument. The Gaia Theory is the idea that all matter works together to sustain life. A problematic metaphor to say the least, it means jumping from the empirical truth that all matter co-mingles into one complex system to the belief that this system attempts to SUSTAIN life. That's a pretty big assumption.
BUT--and here's where I'm going to have to feed your existential dread (sorry)--as a Stirnerite (more or less), I largely agree with Gray's critique of humanism. To any naturalist, Gray's contention that humans cannot escape their base nature certainly has merit. Like it or not, humans are nothing more than animals, bound by our drive to survive and procreate just like everything else. We are utterly carnal.
But humanism is problematic even outside of metaphysics. The philosophy really boils down to two ideas: the positive worth of human reason and the inevitability of progress. Go ahead and try to argue in favor of human reason if you want. It's about as effective as shouting into a wind tunnel. And what is progress? Improvement? The improvement of whom? For what? Whitman wrote: "All is a procession." Yes, but procession isn't always positive. Again: who will agree on what 'positive' even means? Ideas like these are all subjective.
(Incidentally, I haven't yet decided on his assertion that humans might not have free will. I'll admit that I find the notion of complete determinism tragic and scary.)
But: We live in the world we live in, right? I think we call ourselves anarchists for many reasons, but one of them has to be that we all maintain a certain willingness to stare facts in the face. The problem, in my humble opinion, is that many anarchists stop at their critique of the state and the economy. They don't push themselves to punch holes in the ideas John Gray does.
But it's okay. It's okay to accept that you are merely a highly-evolved biological organism in an amoral, naturalistic environment. It's okay to accept that you are in fact limited by your biological nature because that's all you are. It's okay to accept that you rely on the deaths of other beings to survive. It's okay to accept that progress and the common good and humanism and utopia and communism and capitalism and democracy and freedom and morality and justice and kingships and governments and yes, even anarchism, are all abstract concepts created to ease our mental digestion of the world. They aren't real and are prone to misinterpretation and misuse.
The untrammeled individual--the true anarchist--is beyond such things. Of course we should still use ideas as tools to create a more vital life for ourselves. But we can never place them above us in importance. We can never grant them the power of reality because they are only words and perceptions; smoke and mirrors.
So Professor Gray has destroyed one of your idols. Good. You are now more clear-eyed and more practical. Be proud. Instead of trying to rebuild, embrace what's left. A better life awaits you if you stop viewing the world through the muddied lens of ideals like humanism and start weighing your decisions and beliefs on the only scale you can trust--hell, on the only scale you've got--your ego. Your ego is that part of you that lives outside of language and philosophy and classification. It just is.
I'll let Stirner sum up: "The man is distinguished from the youth by the fact that he takes the world as it is, instead of everywhere fancying it amiss and wanting to improve it, i.e. model it after his ideal; in him the view that one must deal with the world according to his INTEREST, not according to his IDEALS, becomes confirmed."
Be confirmed. Be an adult. John Gray is a little off, but at least he's trying.