The net gain of energy in glycolysis from one molecule of glucose is

2 ATP

Explanation:

glycolysis is the beginning of cellular respiration and it yielded net ATP of two produced from 1,3 bisphosphoglycerate conversion to 3-phosphoglycerate (2 ATP) and conversion of phosphoenolpyruvate to pyruvate (2ATP). One will recall that 2 ATP had been expended in the conversion of glucose to glucose-6-phosphate, and in the conversion of fructose-6-phosphate to fructose - 1,6 - bisphosphate. Other product of glycolysis are 2 NADH, 2 Pyruvate that may go into kreb cycle for further energy production depending on the organism type of respiration.