In Excel 2010, the Gammadist function has been replaced by the Gamma.Dist function, which has improved accuracy.
Although it has been replaced, the Gammadist function is still available in Excel 2010 (stored in the list of compatibility functions), to allow compatibility with earlier versions of Excel.
The Excel Gammadist function returns the Gamma Distribution, which is frequently used to provide probabilities for values that may have a skewed distribution, such as queuing analysis.
The user can specify whether the cumulative distribution or the probability density function should be used.
The syntax of the Gammadist function is :
where the function arguments listed in the following table:
x    The value at which you want to evaluate the function (must be ≥ 0)  
alpha    A parameter of the distribution  
beta   
A parameter of the distribution Note: if beta=0, then this specifies the Standard Gamma distribution 

cumulative   
A logical argument that specifies the type of gamma distribution to be calculated. This can have the value TRUE or FALSE, meaning:

The above chart on the right shows the probability density function for the gamma distribution with the parameter α = 3 and the parameter β = 2.
If you want to calculate the value of this function at the value x = 6, this can be done by the Excel Gammadist function, as follows:
This gives the result 0.112020904.
The above chart on the right shows the Cumulative Gamma Distribution with the parameter α = 3 and the parameter β = 2.
If you want to calculate the value of this function at the value x = 6, this can be done by the Excel Gammadist function, as follows:
This gives the result 0.576809919.
Further examples of the Excel Gammadist function can be found on the Microsoft Office website.
If you get an error from the Excel Gammadist 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 