The eight inner gates use up the bodies energies by converting it pure power
the body is made of psychical and spiritual energy which is what is molded and formed into chakra.
Rock Lee relies on them because it powers up his only way of fighting and it just happens that his particular style is how you usually obtain such power in the first place
I don't thinks it possible... Gaara has never shown Taijutsu abilty in any way shape or form. In actually pretty sure he can't even do Taijutsu with the gourd on his back