He wrote a book which is probably full of criticisms of transhumanism, called The Immortalization Commission: Science and the Strange Quest to Cheat Death. But I haven't read that. The criticisms show up in his 7 Types of Atheism. You can also find articles on his view of transhumanism:

https://www.theguardian.com/books/2011/jan/08/john-gray-immortality

https://www.vice.com/en/article/exmj3e/john-gray-freedom