GAMMA.INV Function
Basic Description
The Excel Gamma.Inv function returns the inverse of the Gamma Distribution. The function is new in Excel 2010 and so is not available in earlier versions of Excel. However, the Gamma.Inv function is simply a new version of the Gammainv function, that is available in earlier versions of Excel.
Syntax: GAMMA.INV( probability, alpha, beta )
where the function arguments are:
probability  –  The probability value (between 0 and 1), for which you want to calculate the inverse of the gamma distribution 
alpha  –  A parameter of the distribution 
beta  –  A parameter of the distribution Note: if beta=0, this specifies the inverse of the Standard Gamma distribution 
Gamma.Inv Function Example
The charts below show the Cumulative Gamma Distribution and the Inverse Cumulative Gamma Distribution, with the parameters alpha = 3 and beta = 2.
Cumulative Gamma Distribution Function with α=3 & β=2

Inverse Cumulative Gamma Distribution with α=3 & β=2

The Excel Gamma.Inv function can be used to calculate the value of this function for a given probability. For example for the probability 0.5, the function is:
=GAMMA.INV( 0.5, 3, 2 )
which gives the result 5.348120627.
Gamma.Inv Function Errors
If you get an error from the Excel Gamma.Inv function this is likely to be one of the following :
#NUM!  –  Occurs if either the supplied probability value is < 0 or ≥ 1 or the supplied alpha ≤ 0 or the supplied beta ≤ 0 
#VALUE!  –  Occurs if any of the supplied arguments are nonnumeric 
#N/A  –  Occurs if Excel fails to converge to a solution after 64 iterations. 