Tri-state output circuitry as used for most computer busses can be set Low, High or HiZ. In fact many circuits actually have 4 states as they can be set as Inputs. The state of a bus (consisting of a set of grouped pins) is typically controlled by one or more enable pins.
The GPIO has no similar setting; Outputs can be Low or High; they can alternatively be set as Inputs (with optional pull). Each GPIO is independently controlled.
When set as Input the GPIO is effectively in a high impedance state. All that is needed is to set the pin to Input with no pull.
The Pi actually can be configured into a kind of bus - the SDIO is a 4 bit wide bus with a CLK and CMD pin which is used for bi-directional communication e.g. to SD Card.
The I²C interfaces are also a kind of 1 bit bus, normally with open drain circuitry but simulated on the Pi by configuring as Input.