Attached Link: http://www.runtimedna.com/forum/showthread.php?38538-gamma-script-problem.&highlight=changeGamma

I'm pretty sure it was included in Poser Pro 2010. In the original Poser Pro it didn't work at first, but they fixed it later.

Maybe you could try to reinstall and see if it appears then?

Or, there is a version posted at RuntimeDNA. It is for the original Poser Pro but it is almost identical to the one I have in PP2010.