Air-sea temp difference

[P.R.]

Hello,
General meteorological question:
I need to calculate the 'air-sea temp difference', using the most recent SST
and GFS data, and then export this data to a binary file.

I don't know which temperature field from the GFS model should be used to
calculate this difference, (from the GFS, should I use the '2 meter air
temp',the 'surface air temp',etc. ???).
Does anyone have experience with generating this type of data (the 'air-sea
temp. difference')???

Also, any tips on how to quickly generate this difference within grads would
be appreciated.

Please help,
Thanks,
P.Romero