Re: How does the GDP calculation measure Barter?
See the example of the two female police officers, both with young children. Very sensibly, they put their heads together, and arranged their shifts so that one could care for the other's children whilst the other was at work.
The taxman got involved.
On a wider note about barter, you could probably do worse than listen to this ten part ( 15 minutes per episode) series "Promises, Promises: A History of Debt" http://www.bbc.co.uk/programmes/b054zdp6/episodes/player
You should note that if I want one of your pears, but I only have a live chicken, sorting out change might be tricky (or at least messy and noisy!). Think of a few more examples, and you'll appreciate that debt arose hand-in-hand with bartering.