This would work for the current selected figure:

value=1.0
for mat in poser.Scene().CurrentFigure().Materials():
        shdrTree = mat.ShaderTree();
        if shdrTree!=None:
                for node in shdrTree.Nodes():
                        if node.Name()=="PM:Gamma":
                                node.Input(1).SetFloat(value)

The attached script allows to set the gamma for the current material, actor or all materials and to do it for maps that should not be corrected (Displacement, Bump...), too. It's for Poser Pro 2010 only, though.