During the redox reaction in glycolysis, which molecule acts as the oxidising agent?

The oxidation of glyceraldehyde 3-phosphate (G3P), a three-carbon sugar, yields energy. In this oxidation, electrons and H+ are transferred to NAD+, forming NADH, and a phosphate group is attached to the oxidised substrate. ATP is then formed by substrate-level phosphorylation when this phosphate group is transferred to ADP.