Q.Can you please explain the procedure of ballasting by gravity and what does it mean?

Ballasting 'by gravity' means the ballasting of tanks without involving of any pumping units.

If you have two tanks connected by a piping system that contain water in different height (while they are isolated by a respective valve) and you then remove the isolation between them, water will flow from the one with the higher water level to the one with the lower one, regardless of the shape and volume of the tanks. This is called 'the principle of communicating vessels' and it is based on gravity (and pressure) effect.

If you instead 'communicate' one tank with the sea level - instead of another tank with a higher fluid level - (through a respective sea chest that provides access to the sea water through vessel's hull) provided that sea water surface is at a higher level than some part of this tank, the water will flow in until it gets in balance with the sea level.

You can use the same principle to do the opposite (de-ballast a tank by gravity) if your tank is filled with water at a level higher than the sea level, and let it flow out until it reaches balance with sea level (in this case your tank can be even higher than sea level e.g. top side tanks which then will allow the tank to be almost empty)

Of course, while the water level difference becomes shorter by time (ballasting or deballasting) the flow rate dicreases accordingly.

You should always carefully monitor the in-flow or out-flow rate, first to not exceed the designed flow capacity of your piping (for safety reasons) and secondly to not fall much under than pumping capacity rate (because you will be losing time to complete operation)