depreciation in income approach of GDP I have some doubt regarding gdp sector, while measuring gdp by income approach, we add depreciation. My doubt is: does the cost of goods that go into replacement of existing worn out capital is only treated as depreciation? Or do we also add the depreciation expense of all existing and serving goods, which have not worn out?
In theory, depreciation is a reduction of the capital stock. This includes capital assets that have actually been worn out and also capital assets that are not usable any more for any other reason (conceivably old machinery after upgrading to a new technology or so). 

In practice, macroeconomic datasets infer depreciation either from what the firms in the economy in question say in their accounting or from changes in the capital stock that are not explained by any other influence (investment, dissolution and selling of the firm, ...); this is data that also comes from the firm's accounting. Note that this is not necessarily the same as the actual depreciation according to the theoretical definition since there may be any number of reasons for firms to write off capital assets early or late or at any given time. The formal rules for what practices in terms of write-offs are lawful or not are quite complicated and differ between legal systems (i.e. between countries).

Further, macroeconomic models sometimes simply assume a fixed depreciation rate which they then proceed to compute from averaging over the time series constructed from investments plus the differences in capital stock and dividing by the capital stock. This is, of course, not the way it should be done but nevertheless ...