Saleable area or "net" size or SA is the total square footage including bay windows and balcony. However legally they cannot include platforms, terrace or rooftop. Some agents and landlords do bundle terraces or rooftops in their saleable area advertisement to gain an illegal advantage in attracting your enquiry. This is a violation with the EAA and could be reported if you see abuse by particular individuals.
I am not sure if saleable area is calculated from the outermost, midpoint or innermost point of the wall.
Gross area is the total square footage of the estate including the common areas. It doesn't necessarily reflect the size of the flat but it gives you an idea if the estate is larger and potentially has facilities or not.
Some people say the flat has better efficiency. This means the saleable area and gross area are close to each other. There is usually a percent. Let's say 70% is very common. So if you find a flat with 90% efficiency that means there is probably a small or no lobby and no facilities. If you find something with 50-60% efficiency then there should be a full clubhouse, podium, nice lobby etc.
However I do not believe the new flats are advertising gross area anymore, rather only saleable area. When I was a newcomer I was annoyed about gross area and thought it was "cheating" but after living here a long time and currently in the flat search phase again, sometimes you desire a building with certain minimum space as it is how you are welcomed home everyday. So be open minded and try to understand the different viewpoints is key.