"Gamma" simply refers to the shape of the curve that maps sensor readings into image brightness numbers. A linear curve simply scales the sensor values, a logarithmic gamma curve means more values are attributed to the shadow region, preventing a loss of detail and increasing the dynamic range you can exploit in post production.
If you shoot raw you're capturing 14bits of luminance data from the sensor and you can apply whatever gamma correction you like during the raw conversion so you don't gain anything from a nonlinear tonecurve.
Dynamic range is the difference between the saturation point and the point where detail is lost to noise in the shadows. By carefully choosing the definition of acceptable detail manufactures can massage almost any DR figure they want out of the data! Canon saying "12 stops" is meaningless without knowing what threshold they've set for when noise overcomes detail.