GAFA is an acronym for Google, Apple, Facebook and Amazon (the second and fourth companies are sometimes reversed in order). The acronym serves to identify the dominant companies as an entity -- effectively an oligopoly that controls much of the tech industry market. GAFAM, adding Microsoft to the list, is a common variation on the term.
The term GAFA is more commonly seen in Europe, where it is often mentioned in the context of litigation or investigations. In the United States, Google, Apple, Facebook and Amazon are more often referred to as the Big Four tech companies (or the Big Five when Microsoft is included). Eric Schmidt, Phil Simon and Scott Galloway began referring to the companies as the Big Four as a reference to their disruptive effect on technology and culture, which is what makes them stand out from other large tech firms. In any case, the terms were chosen to identify the entity as a force to be reckoned with.
The European Union has stricter controls on big business than the United States and more concern for consumer rights. As a result, GAFA firms are frequently under investigation there for anti-competitive practices, collusion, tax avoidance and user data gathering.