Ok, so what can he do? I mean, let's say he spends 10 billion dollars, on 100 million people. That would be $100 each. I don't suppose there is any way to insure that all these people get their $100, because of the problem of distribution, identity verification, etc. You can't mail them a check. You can't mail them cash. You can't trust local delivery men to do the job (they'll pocket it and say they delivered, or they'll get robbed).
If you try to provide a service, you'll get knocked down by the bureaucracy. Look at the fiasco around providing identification cards for the poor (see
https://en.wikipedia.org/wiki/Unique_Identification_Authorit...).
In my opinion, the best way to help the poor in India is to help India become a more prosperous nation, and as such, help the development of education, industry, public safety, environmental safety, research and development, and cultural identity within diversity. This is something the governments of India (national, state, and local) have generally pursued vigorously since independence more than 60 years ago. That the problem with poverty and illiteracy is still so pronounced only demonstrates the magnitude of the task.
Look what a long letter I write... I would suggest that the only way to help the people of the slums is for the rest of the people of India to want to help the people of the slums. There has to be a mindset that the majority of people in India adhere to, namely, that poverty is not acceptable, and that the factors that enhance poverty are to be remedied, even if this means pushing aside long-held traditions and ways of doing things.
Which is a tall order, even for Bill Gates.
So instead he goes after specific problems, such as polio, and it's all good, don't take me wrong, but, huh, it's not nearly as impressive in the grand scheme of things as it's made out to be.