On Parole it is possible to set custom brighness/contrast/hue/saturation settings but not the gamma. It would be better if gamma can be modify too.
I'm not sure why/when you'd need gamma. To be honest I never use any of those other image-settings and don't really see why they're needed (but they're already there, so for now we'll keep them).
As we couldn't find a good reason as to why to implement this (and no-one else provided one), we're closing this as wontfix.