There is actually a pretty darn simple way of doing gamma correction. Basically, you have an empty scene with a simple set of shaders on the background. (I got this from Bagginsbill, so it should be in a thread somewhere.) Open up the material room, and select the image you want to correct as the image in the "image_map_2" node. Select the render dimension to be the same as the original image. In the render options, use the simplest you can. (You don't need shadows etc.) However, you might want to set the post-filter as you like. (It is a lot easier to post filter at this stage than in the original render.)

I'm attaching the basic scene for gamma correction. Although this is the format of a .txt file, it is actually a .pz3 scene file.