How does glycolysis produce ATP?

Glycolysis produces energy through the form of ATP. ATP is created directly from glycolysis through the process of substrate-level phosphorylation (SLP) and indirectly by oxidative phosporylation (OP). The three stages of glycolysis are phosphorylation of glucose to glucose-6-phosphate (G6P) which requires ATP, production of triose phosphate (TP) and oxidation of TP to pyruvate, which yields 2 reduced NAD molecules (NADH) and 4 ATP per glucose. 2 ATP molecules were used in the first stage so net ATP gain is 2 ATP. This is substrate-level phosphorylation.  Indirectly, ATP is produced through oxidative phosphorylation. This requires the electron transport chain (ETC) and ATP synthase found on the inner mitochondrial membrane. Briefly, in the ETC the protons from reduced NAD are pumped across the membrane, using the transfer of electrons as an energy source, (from the inner mitochondrial matrix to the intermembrane space) to produce a chemiosmotic gradient. This chemiosmotic gradient provides the energy to drive formation of ATP from ADP and inorganic phosphate within the ATP synthase enzyme.