The gear pump may have a casing, a main shaft and rotating means connected to one end of the main shaft and configured to rotate the main shaft. Housed by the casing, a toothed wheel and at least one other toothed element are intermeshed with the toothed wheel. The other end of the main shaft is connected to one of the toothed wheel and the casing. Rotating the main shaft relative to the rotating means rotates the toothed wheel and the at least one other toothed element relative to the casing, thereby generating the flow. The other one of the casing and the toothed wheel is configured to be rotated around an axis defined by the main shaft for varying a flow rate of the generated flow.