After the India Ink dries completely, you take a sprayer and wash everything off - the ink and the white gouache. You can do this outside with a garden hose, under the sink with the kitchen sink sprayer, or in a tub of water (which you have to change about 3 times because it gets grey and yucky pretty fast). Any way you choose, wash off all the ink and gouache you want, leaving some ink behind (or a lot, depending on your preference).
And then you have this.
The white gouache has allowed the white of the paper to show through and the India Ink has gone into all the areas you left without gouache, making a nice print-like painting. (I used Daler Rowney white gouache on this one, something I had leftover in a tube; and the white after everything was washed off was a dull greyed color - so I think Winsor Newton gouache gives the best result if you want your whites to be clean and bright before putting color on at the end.)
At this point, you can leave it like this, or go back with your watercolors and paint in some color. (You want to make sure the paper is dry before painting on it and that may take a while since you're using 300# paper and it's pretty soaked after washing off all the gouache and ink. So I'd give it another 4-5 hours to dry.)
When my paper was dry, I went back with my watercolors and gave the goldfinch some color, even in the background area (a pale wash of brown from a mix of 3 primary colors).
I hope you enjoyed the demo and will give this a try. It's really easy. It just takes time for each layer to dry. If you don't like waiting, you could work on 2-3 of these at a time.