GCASA is the Global Community Aeronautics and Space Administration, presumably formed out of NASA, Roskosmos, ESA, JAXA and other space agencies.
The agency has a hierarchical structure under a single leader, who is occasionally interviewed on TV to explain or predict asteroid impacts, but the leader is never mentioned by name or pronoun, therefore their identity is entirely unknown to the readers.
GCASA plays a role in monitoring extraterrestrial events, but are shown to be very poor at it, missing any warning for several meteor showers and only identifying upcoming major events such as the fall of Wormwood hours before the impact. Since space exploration is not mentioned anywhere in the series, it's reasonable to assume that Nicolae Carpathia severely cut the budget of the world's space agencies when he combined them, put an incompetent in charge of the global agency, or both.
GCASA is mentioned in Soul Harvest and Apollyon in the adult series, and Darkening Skies and Attack of Apollyon in the kids' series.