I think an easier way to do this would be distillation not GC. the IR peaks from methanol and ethanol are similar so I don't see you having great success in distinguishing your fractions out of the GC.

You should also be able to do this with the density ( which would probably be easier than distillation) Methanol and Ethanol have different densities so you should be able to weigh out a volume and determine the contribution from each species.

good luck

If all you want is a simple determination to know hoy much it is you can inject a dilution of your mixture, and then a dilution of neat metanol, both of them you have to know the initial quantity... then you can relate the ratio of peak areas: Sample Methanol area/reference metanol area = metanol in sample / metanol in reference.

http://www.chromatography-online.org/quant/Reference-Standards/GC-and-LC/External-Standard-Method.html

If you want a well standarized determination you could make a curve, using standart additions or even an external standar curve to known the error of your determination, if your device is wel calibrated they would have a calibration certificate and you don't have to make any curve.

This could help too.

http://www.tau.ac.il/~advanal/StandardAdditionsMethod.htm

If they used that method, they just know the concentration of methanol in ethanol. not the mole of the methanol. are they weigh the mixture first??

Yes. You'll obtain a concentration then for know the mole or mass of a component you have to know the total mass of the mixture.