Explain how in glycolysis the splitting of one glucose molecule results in 4 atp but there is only a net gain of 2 atp.

In Glycolysis, there’s consumption of two ATP molecules and production of 4 ATP molecules. So the Net gain is 2 ATPs.  

Glycolysis is the step of respiration which is the 1st step of aerobic respiration. Here one ATP is consumed each at the steps of Hexokainase and phosphofructokainase. So these ATP's are required to convert the corresponding substrates into its phosphorylayed products.

Then 2 ATP's each are produced in the steps of phosphoglycerokainase and pyruvate kainase. So there are utilisation of 2 ATPs and production of 4 ATPs. So the net production is 2 ATP.