Not the OP, but Wikipedia [1] lists the number as 25-35%, listing this book [2] as a source in which page 261 seems to provide numbers ranging from 32% to 24% in the centuries leading up to colonization. It's not clear what the source for book's estimates are, although some explanation seems to be provided for the author's methodology.

The same Wikipedia article also lists another text [3] as a source for the historical share of GDP table [4].

[1] https://en.wikipedia.org/wiki/Economic_history_of_India#cite...

[2] https://books.google.co.uk/books?id=rHJGz3HiJbcC&pg=PA257#v=...

[3] https://archive.org/details/contoursworldeco00madd

[4] https://en.wikipedia.org/wiki/Economic_history_of_India#GDP_...