Anamorphic correction:

In GMOS-N, the unbinned CCD scale is 0.072" per pixel.
For the B1200 l/mm grating, inverse dispersion is 0.23 Ang/pix
and a 0.5" slit would give R=3744, FWHM(pix) = R/(lambda)*dispersion
projected slit width: FWHM(0.5" slit) = 4.06 unbinned pix * (4000 Ang/lambda)

The Anamorphic correction at the central wavelength is given by:

A = Cos(alpha)/Cos(beta)   ; see alpha, beta definition below

GRATING = 'B1200+_G5301'       / Grating name                                   
GRWLEN  =                 440. / Grating central wavelength (nm)                
GRORDER =                    1 / Grating order                                  
GRTILT  =              48.0647 / Grating tilt angle (degrees)          

for the example above this is:  A = 0.67 
so this would predict a 0.5" projected width of 0.67/(0.5"/0.072") = 4.65

To find the wavelength dependence, we can use:

\alpha = (90 - GRTILT)
\beta  = (alpha - 50) 
lambda_central = d*(sin(\alpha) + sin(\beta))  where d=1/(1200) mm
               =  439.99 nm, for the example above.