When using GSuite it is used in a browser, and this is key to the Google strategy. The goal is to get more users spending more time in their browsers, in order to funnel them to Google Ads and to serve Google their browsing data.

GSuite exists, at least in part, to break down the model of the walled corporate intranet. There are just too many people behind the corporate veil to ignore; and as a bonus, the browsing and cloud app usage habits they develop at work will tend to translate to their usage habits at home.

Also, as noted by another responder, GMail has had ads since launch. I was a beta user from day 1, and I recall the ads.

Edit: worth noting the historical difficulty of getting open source software into the business environment because corporate purchasers were wary of anything that was "free"; putting a price on something makes it more palatable to corporate adoption.