This work is a monumental analysis of the Christian faith by the man who is widely recognized as the greatest Christian theologian of all time. This work is huge and took me 2 years of daily reading to get through it. But it is only the second most widely known of Aquinas' works. His best known work, the Summa Theologica, is more than twice as long as the SCG and is an insanely comprehensive treatment of the entire range of Christian theology. The difference between the ST and the SCG is that the ST is a work of Sacred Theology in which Aquinas makes heavy use of scripture and the works of the early Church Fathers to illustrate and prove his theological points. In the SCG, Aquinas does use scripture and the Fathers to a degree, but the main thrust of his arguments are from the secular philosophers, especially Aristotle. Because of this, most scholars refer to the SCG as a work of "Natural Theology", because in it Aquinas uses wisdom that was not revealed by God through the prophets or the Fathers to make his points.
Natural Theology is a great tool that scholars can use to illustrate the truths of the Christian faith to those who are not Christians and therefore do not accept the authority of scripture or the Church Fathers. The title of this work, "Summa Contra Gentiles", means a "Summary Against the Gentiles" because the intended audience of this work is not his fellow Christians but rather non-Christians; Muslims, Jews and heretics. Some scholars speculate that Aquinas' intent was to use the SCG as a handbook for missionaries to foreign lands. But having read this work, I find it hard to believe that this is the purpose of the work, since I would think that Aquinas would have made more reference to the actual teachings of Muslims and Jews than he did if this were his intent. Rather, my belief is that Aquinas meant this work to be a counter-arguement against many of the works of philosophy that were written by non-Christian writers; not only the ancient pagans but also Muslim scholars like Avicenna and Avveroes, and the Jewish thinker Moses Maimonides. These writers were growing in popularity during the lifetime of Aquinas, and Aquinas does actually refer to the works of these latter writers, especially in the first two books of SCG.
Unlike the ST, which had a point-counterpoint format in which a Aquinas proposes an erroneous thesis and then proves his actual thesis by arguing against the erroneous one, the SCG has a straighforward paragraph format in which Aquinas lays out his teachings and explains why they are true. Ironically, considering that this work is explicitly written against a party, Aquinas' approach is much more a form of positive argumentation than the ST, in which he argues against the counterpoints that he proposes at the beginning of each article. Because of this, the SCG is really a great source of straighforward teaching about Catholic theology than the ST, in which the reader must weed through lots of counterargumentation and contrary opinions. The ST, in contrast, is better for the apologist who can familiarize himself with the opinions that oppose the orthodox Catholic teaching and how to refute those opinions. So, my opinion here is that both works are essential for the Catholic theologian, although in different ways and for different reasons.
The SCG is really one of the most valuable and underappreciated works of Catholic theology in history. I got my copy of this work on the Kindle for about $3, so there is really no excuse why more Catholics don't get their hands on this work and study it. Granted that the work of Aquinas is not light reading, and it takes a lifetime or more to really wrap your mind around his thought. Still, it is essential for the serious Catholic to at least try to understand Aquinas and this work.