From first looks it seems that gamma is already applied to the materials.  Your gamma corrected image reminds me of my renders when I apply a gamma setting to the materials and then use gamma correction.  There is a script in Poser 8 and Poser Pro 2012 that will allow you to change the gamma settings for the materials.

As for the what setting the gamma should be on, then from my point of view the answer will be. Leave it on any setting that you wish to have it on.  In other words have it set at what you think is best.

If you want gamma correction to be set at 2.2 then you'll have to make sure that all of the other settings that you want to use be set at 1 (If I remember correctly).

When I look at your render I can't help but to think that you are correcting materials that already have a gamma setting.  

Here is some information I wrote about Gamma Correction.