http://www.youtube.com/watch?v=foAAVQKvhtA
some unknown person whose narration I do not understand put up the video of Fong sifu doing the slt a long time ago.
Gan sao is at 2.56
The gan is also there in biu jee as gan/jum and in the jong as a section with gan combined with kuen. There is also gan do with the bjd.
Good Ip man wing chun usually involves cooperation with both hands- so there is always something else balanced with the gan.
You can also see Ho Kam Ming's later protege Lui Ming Fai with the gan in the forms and the knives.
The gan can be practiced with the rich variety of footwork that we use.
The gan is usually used for the lower gates. For development the arm cam be straight for learning how to release energy. In application it is bent as a precaution against arm bars.
The power of the gan comes from coordinating the whole body
In attacking it can aim at the mother line via the groin, the kua the kidneys etc.In defending it can control a low attack while a jum attacks the neck/throat.In certain occasions the gan can help witha throw or even an elbow break.
There is more but I hope the summary answers your question.