"So, does it mean that on takeoff acceleration and landing braking deceleration, the G meter will register 1,0, because the aircraft vertical speed and acceleration are zero and the accelerations along fuselage are not counted?"
Yes.
"And that during steady horizontal cruise in a slightly nose-up attitude, the g meter will register less than 1,0?"
Yes.
"Yes Dick, you can calibrate your g meter as you like. For convenience, subject to normal surface gravity, you can make it 1.0. Out floating in free space subject to no thrust, you can make it zero. Then you can take that meter and put it on the surface of any planet to get a good reading of gravity relative to the Earth's surface, or stick it on an aeroplane in Earth's atmosphere in a steady climb, cruise or descent, with no acceleration effects in which case it would read 1.0!"
If the local gravitational acceleration is 1g and, assuming as is the norm a one-axis g meter, if the g meters sensing axis is aligned with the local gravitational field.
In the case of an aircraft which is not level, the g meter will not be aligned with the local gravitational field and will thus only register the component of local gravity parallell with the g meters sensing axis. Hence it will read less than 1 g.