Like other word processing software, Google Docs has been checking spelling and grammar for some time. However, Google now differentiates itself by adding grammar suggestions based on machine translation, going head-to-head with third-party services such as Grammarly.
Introduced for the first time in July 2018 via an early adoption program, Google's grammar suggestions use an automated translation model designed by linguists. Grammar is a complicated subject, with the blog post mentioning that linguists disagree on whether a suggestion is correct once in four, so it's nice to see Google make real efforts. Suggestions appear as wavy blue lines under the wrong words / phrases, as you can see in the GIF above.
Google says the suggestions will reach G Suite Basic, Business and Enterprise customers, so basically everyone. It is not clear when exactly this feature will be deployed; it seems not to be live on any of my accounts.