The Excel Gamma.Dist function returns either the cumulative distribution or the probability density function for the Gamma Distribution. This distribution is frequently used to provide probabilities for sets of values that may have a skewed distribution, such as queuing analysis.
The Gamma.Dist function is new in Excel 2010 and so is not available in earlier versions of Excel. However, the function is simply a new version of the Gammadist function, that is available in earlier versions of Excel.
The syntax of the Gamma.Dist function is :
where the function arguments are:
x    A positive value at which you want to evaluate the function  
alpha    A parameter of the distribution  
beta   
A parameter of the distribution Note: if beta=0, then the Standard Gamma distribution is calculated 

cumulative   
A logical argument that specifies the type of gamma distribution to be calculated. This can be either:

The above chart on the right shows the probability density function for the gamma distribution with α = 3 and β = 2.
The value of this function for a selected value of x can be calculated by the Excel Gamma.Dist function. For example, the function value at x = 6 is given by:
which gives the result 0.112020904.
The above chart on the right shows the Cumulative Gamma Distribution with α = 3 and β = 2.
If you want to calculate the value of this function at x = 6, this can be done with the Excel Gamma.Dist function, as follows:
=GAMMA.DIST( 6, 3, 2, TRUE )
This gives the result 0.576809919.
Further examples of the Excel Gamma.Dist function are provided on the Microsoft Office website.
If you get an error from the Excel Gamma.Dist function this is likely to be one of the following :
#NUM!   
Occurs if either:

#VALUE!    Occurs if any of the supplied arguments, x, alpha or beta are not recognised as numeric values 