gamma.dispersion {MASS}  R Documentation 
A front end to gamma.shape
for convenience. Finds the
reciprocal of the estimate of the shape parameter only.
gamma.dispersion(object, ...)
object 
Fitted model object giving the gamma fit. 
... 
Additional arguments passed on to gamma.shape .

The MLE of the dispersion parameter of the gamma distribution.
Venables, W. N. and Ripley, B. D. (2002) Modern Applied Statistics with S. Fourth edition. Springer.
gamma.shape.glm
, including the example on its help page.