This is rewriting history. GMail was created by Google engineers (as an internal only project) way back in the day because they wanted a better email client (Gmail was, originally, a 20% project). Eventually they added ads because well, sure, companies want to monetize things. But to say that it exists
solely to show you ads is a disservice to the people who originally created the tool. It was not made to, and does not exist to show ads. It was made to, and exists to be a better email client. To do that sustainably, it now show ads.
Also I'm pretty sure you can disable all promotions within Gmail, and I'm fairly sure that Inbox has, to my knowledge, never shown ads. So as billboards go, I think it's anything but brilliant, since it has never shown me an advertisement!
Much like how CoLab[1] apparently exists solely to show you ads and how hangouts and Google Voice are there just to show you ads (I'm not sure where though). Duo is just a video chat app to show ads. Clearly.
This argument is so silly. Indeed there are people at Google whose job it is to do things with ads. That doesn't mean that everything that Google does for free is intended to be monetized with ads (see, again: CoLab, which gives away free GPU compute). There are a variety of reasons for a company of Google's size to make products freely available. Showing ads is one of those reasons, but by no means the only one.
Note: I work, tangentially, on Gmail.
[1]: https://colab.research.google.com/notebooks/welcome.ipynb