This is the expected behavior in MATLAB 7.8 (R2009a).
As per the documentation for the GAMMAINC function in MATLAB 7.8 (R2009a) , the function is not intended to work with complex inputs. The fact that that worked in MATLAB 7.7 (R2008b) was unintentional. In the latest version of MATLAB, hard error checking has been added and the error message is intended.
In earlier versions if you were getting answers that were satisfactory, a workaround would be to make a local copy of the code from the earlier version. However, note that this is not supported.