Can you explain a little bit more why you need this? That's sounds strange to me. If you measure concentration of GSH in Media (or any other liquid) you can normalize it by diving to volume you've used therefore you will get mol/liter. If you measure it into cell total lysate you can perform a BCA analysis to get a total amount of protein that you've got (in grams) and you can normalize it by dividing to that amoint e.g. mol/g