With the ffb the only gamma options are 1.0 or 2.2, attained by using ffbconfig to set linear or non-linear modes, respectively. These gamma values are not adjustable like the gamma values are for Leo and GX framebuffers. This is with the non-linear visual with a hardwired gamma value of 2.2.
On ffb you are limited to either gamma enabled or disabled. If enabled the value is always 2.2. If an application is calculating its own colors (ie, it isn't doing lighting) then it can tweak the colors to get a gamma effect (it can artifically brighten them), but apart from that there's no other way - the gamma values are hard-coded into the LUT.