In short
Gross area is the total area of a unit calculated including wall thicknesses and a share of the building's common areas. Net area is the internal space actually used. The same apartment can be described as two different figures under these two measures.
The gross calculation includes the unit's own walls, half of the shared walls and the share falling to it from the building's common areas. The net calculation shows only the usable space within the walls. The gap between the two varies by project and is usually between a fifth and a third.
Listings and sales presentations mostly use the gross figure because it looks larger. The problem is not the figure itself but the comparison: a price per square metre comparison in which one project quotes gross and the other net is wrong from the start.
Why it matters to an investor
Price per square metre is the only common measure an investor uses to compare two projects. When that measure is not built on the same definition, the comparison produces nothing. Asking which figure you have been given is a single sentence, but a decisive one.