Is Van der Waal's force the real source of friction?
While van-der Wall forces contribute to the frictional force, they are not the only reasons. Other secondary inter-molecular forces like electrostatic attractions, polar interactions, hydrogen bonding and more also play a significant part. Apart from this features on the surface of materials, present as abrasions are also responsible for locking together and resisting change. 
The case of a gecko sticking to wall is not just about these forces and interlocking, but more about how the wrinkles on feet of a gecko act as several small vaccum cups which provide it the stability of sticking upside down as well(think how friction plays almost no role here)