The Gamma Distribution is frequently used to provide probabilities for sets of values that may have a skewed distribution, such as queuing analysis.
The function is explained in detail on the Wikipedia Gamma Distribution page.For a set of supplied parameters, the Excel Gamma.Dist function calculates the value of either the cumulative distribution or the probability density function for the Gamma Distribution.
The Gamma.Dist function is new in Excel 2010 and so is not available in earlier versions of Excel. However, the function is simply an updated version of the Gammadist function, which 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 using the Excel Gamma.Dist function, as follows:
This gives the result 0.576809919.
For further details and examples of the Excel Gamma.Dist function, see 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 either:
