Picking from the discussion, I haven't read the whole discussion.
In nominal gdp terms the US, India mismatch will be undone in 30-35 odd years from today, at least that is what the predictions are.
As the things are panned out today, we could do it a lot earlier since we have been doing a lot better than predicted and the US on the other hand is doing a lot bad, but then with the political leadership we have and future being uncertain, that it is, nothing for sure can be said.
While calculating nominal gdp three factors are important:
Real growth rate.
Inflation rate.
Exchange rate.
While calculating indian gdp figures, please keep in mind the real growth rate to be between 7-9%, over an extended period of time the same ideally be taken as 7%, inflation at around 5-7%, and exchange rate will have to come down in 30s, if not less, since indian currency is already a lot more devalued than it should be and is bound to appreciate in the long run which will positively reflect on nominal gdp.
For the year 2020 recent revised predictions for India have been 4-5t usd (nominal gdp), which is what I like to go with, but then mukesh ambani has put the figure at 9t usd. That said, India has recently doubled gdp in last 4 years (2006-2010), and if that is what the trend remains, then by 2020 the nominal gdp could be closer to what mukesh ambani is suggesting, though the rider comes in the form of inflation, which as you would agree isn't a good thing.
Nominal gdp growth rate last fiscal was somewhere around 20(+)% and not 9%, don't have the exact figure.
Purchasing par parity was devised to get a hang of purchasing power of a currency in different economies, and how do they equate.
A dollar in US buys a lot less than it does in India. The difference in US dollar terms is to the tune of 2.7-2.8 times, which means in India the same dollar bill will buy 2.7-2.8 times to what it will buy in the US.
The equalizing factor is 2.7-2.8 times, which is when US dollar purchasing power parity gets drawn but in the long run this factor will only reduce, which mean a US dollar purchasing power will reduce in India.