How Does GDP Differ From NNW as an economic measure?
I cannot find any reference to a specific "Net National Wealth". But I assume that you refer to the National Wealth, in general (also called Net National Worth).

The GDP has been explained in another question, so I will not come back to it.

The National Wealth is a measure of the total wealth of a country: assets, liability, etc.

To my understanding, the main difference resides on the fact that the NW is a rather static measure, whereas the GDP focuses on the dynamic part. Some examples


the value of a house will appear in NW, but not in the GDP;
if you buy a product from your neighbour, the NW will stay constant, but the GDP will increase.


If a country has a high GDP, its citizens/companies should accumulate wealth, and thus the NW should increase. But as the GDP also contains movement within the country those are not reflected in the NW. A diamond is in NW, the operations to cut it is in the GDP.