In the same way as rest of the word processor, Google Docs presented the grammar and spelling checks a while ago. While, now Google is separating its own by adding grammar suggestions that are machine translation-based, on the whole going opposite to the third-party services including Grammarly.
Google’s grammar suggestions were introduced for the first time back in July 2018 by means of an early adopter program; it makes use of a machine translation model that it does its work by having linguists to design.

As stated by Google, the suggestions are supposed to come out for Enterprise, G Suite Basic and the Business, so basically all the people will get access to it. The time has not yet been made certain as to when this functionality is going to come out.