From the GDP lift caused by lifting large populations out of disease and poverty.

Thought experiment: how fast do you think our scientific knowledge would advance if 90% of the US were living in poverty and spent most of their day simply figuring out how to survive?

Sure, but the only way to lift large numbers of people out of poverty is growth. That's not something you can do by fiat.

You can't do it by fiat, but you can do it by redirecting funds to things that have larger impact on solving disease and poverty, which is what Gates is doing. At the end of WW2, ROK was one of the poorest countries in the world with one of the lowest rates of literacy. Now it produces a large portion of the world's consumer electronics and has top quality research centers, in spite of never solving government corruption that whole time. This is mainly due to how well war reparations from Japan and aid from elsewhere were used to develop the country.