It has to be noted, first of all that this book is now 30 years old. Much has changed since it was written and to that end the author has included a new preface which acknowledges this. He also acknowledges that there are some factual errors within the book but that he would rather the original text be preserved as it was originally written, rather than constantly be revised.
The starting question is this: how could we identify if there is life on another planet? In other words, what are the signatures that distinguish life from non-life? The answer is not that straightforward, though Lovelock, with some acknowledgement given to some other scientists, comes up with a working definition for what characterises that which is living. But what Lovelock then does is to apply these criteria to the whole of planet earth and comes to the startling conclusion that the earth (or at least the biosphere) is a living thing; not just that it contains living things, but rather that it is itself a living entity, which has then been dubbed Gaia, after the greek goddess of the earth.
From here, Lovelock then looks at various aspects of biology and chemistry on earth and seeks evidence for this claim. His central argument is that of homeostasis: that the earth is self-regulating in order to maintain the conditions needed for life.
The book is characterised by two different personalities, so to speak. On the one hand, there is a quite reasonable scientific discourse (mostly focused on chemistry) about the make up and balances within the atmosphere and oceans, while on the other hand there is an impassioned environmental polemic on what mankind has done to harm the planet. While I do disagree, per se, with having these two styles married together, the way it is done seems to take the edge off the level of scientific credulity that Lovelock might have otherwise been afforded. My impression of it was that the scientific overview of feedback systems was immensely interesting, but the overarching Gaia hypothesis was itself unnecessary. Though this book has been hugely influential, particularly within the environmental lobby (rightly, I believe) the weight of scientific evidence for the master narrative is small and yet to be convincing.