Chakra is created when two other forms of energy, known collectively as one's "stamina", are moulded together. Physical energy is collected from each and every one of the body's cells and can be increased through training, stimulants, and exercise. Spiritual energy is derived from the mind's consciousness and can be increased through studying, meditation, and experience.
So, in case of taijutsu, the user uses kicks, punches, etc to fight. To perform these actions the user definitely requires normal energy produced by the cells in the body. Now this part of chakra (physical energy) is required to produce chakra. If the user uses this "physical energy" faster, which is the case for taijutsu, the amount of chakra produced will be less. So the user doesn't use chakra, just its production gets limited. Now in case of the 8 gates also, no chakra is used. Opening the gates removes the physical limitation on the body like more muscle movement.