Without the Google login, Google has a bunch of 32-bit numbers from people’s search activity. With gmail, they now know a lot more about what’s behind those 32-bit numbers.
Let's remember that Gmail opened the door for Google to become the default identity provider for many people. A lot of consumer and behavioral data can be inferred from every service account tied via OAuth to google.
Were you searching for homes to buy? You'll probably use your google account. Bought a couple of home automation devices, cameras, and doorbells? Chances are you are using a google login.
Use Waze in your car? Now google knows what car you drive and the places where you go and can tie it nicely with all other pieces of data because you are using your google account.
This is invaluable for them. From your usage of a google account, they can infer your family arrangements, probably what your job is, your income and wealth level, tastes and habits.
They may be paying to be the first option in "choose your email provider" lists. Although Apple's Mail.app (both iOS and macOS) offers their own and Microsoft's services before Google's.
For Google, the easiest way to gain new GMail users is to not deliver mail coming to existing users from 3rd party services or to throw it into trash/spam folders for no reason.
I’m describing using telemetry correlating which IP addresses/devices have used specific gmail accounts, nothing to do with the contents of those emails.
The gmail account just as a correlation ID has a ton of value in search.
The data from Gmail isn't what's valuable to Google, it's the fact that targeted ads are more profitable and Google can target you with ads accurately because you logged in and identified yourself to access Gmail.
Were you searching for homes to buy? You'll probably use your google account. Bought a couple of home automation devices, cameras, and doorbells? Chances are you are using a google login.
Use Waze in your car? Now google knows what car you drive and the places where you go and can tie it nicely with all other pieces of data because you are using your google account.
This is invaluable for them. From your usage of a google account, they can infer your family arrangements, probably what your job is, your income and wealth level, tastes and habits.