There is a net gain of two ATP molecules produced by glycolysis, according to the University of Illinois at Chicago. The university also says that while four ATP molecules are produced overall, the early stages of glycolysis consume two ATP molecules.
The University of Illinois at Chicago says that glycolysis is performed by every organism on earth, and as such is one of the most common biological chemical reactions in existence. They state that glycolysis takes place in the cytoplasm of cells, rather than in the mitochondria, where more efficient energy-producing reactions take place. According to the University, the Kreb's cycle, which is the next stage in energy production for many organisms, produces two ATP molecules. Oxidative phosporylation, the final stage, which occurs in the mitochondria, produces 32 ATP molecules.