You'd have to figure out a way to make the price feed for this (so basically a way to measure the GDP of all countries in real time...).
While the world is still using currencies like USD (and EUR, CNY, etc.) you don't really need it to be real time. If you can measure it on a monthly or quarterly basis you can superimpose those shifts on top of something like the BitUSD price feed. The value of BitGWP (in BTS) would fluctuate in the short term based on the price of BTS in USD, but it would change over the long term because of the GWP measurements (priced in USD) made periodically and input into the blockchain (see further details
here). Since USD is price stable in the short term, and the GWP adjusts the price of BitGWP over longer periods of time, I think this should give the desired result for BitGWP.
The problem is what happens if the world adopts BitGWP and USD, EUR, etc. disappear? If everyone is getting paid in BitGWP, it isn't clear what should even be put into the price feed. Perhaps at that point the price feed becomes unnecessary since the BitShares exchange drives the price of BitGWP/BTS. But with the price feed gone, only the free market is driving the price of BitGWP and it isn't clear to me how the BitShares network should be adjusting its supply according to the calculation of GWP (priced in BitGWP at that point).