To change the brightness and contrast of front camera

gst-launch v4l2src device=/dev/video1 ! ffmpegcolorspace ! videobalance brightness=0.5 contrast=1.2 ! autovideosink

The "brightness" property

Allowed values: [-1,1] Default value: 0

The "contrast" property

Allowed values: [0,2] Default value: 1


You might need to install gstreamer0.10-plugins-good-extra for the video balance pipeline to be present.

Apparently you can change gamma too but I've not found that yet.

BTW I found all this using power search and searching the terms 'front brightness', just so you know.