Where do remittances come into play in the formula for GDP? In the classic formula for GDP, i.e., GDP=C+I+G+X-M, where are remittances accounted for? With remittances as high as third of GDP for developing countries, I wonder how they are not explicit in this equation.
Gross domestic product (GDP) is the total value of output in an economy, this can be measured only by Output using this formula.

This method uses GDP = C + I + G + (X-M) where

C: Consumption (Household spending)
I: Investments 
G: Government spending 
X: Exports from an economy 
M: Imports into an economy


The remittances that you mention about are not made against any services.  While remittances can be a source of GDP growth by increasing household consumption, it does not directly add to GDP, it does affect GNP though.

For a clear comparison of these two terms GDP and GNP use this

Alternatively you can use incomes to calculate GDP, however note that even in that method you add only those incomes that are made from the production of goods and services are included and not transfers