Google Docs has had a spell check tool for years. The grammar check tool has always been a bit rudimentary. That is going to change in the next few months.
Earlier today Google announced that a new grammar suggestions tool is going to be added to Google Docs. The new grammar suggestions tool will automatically underline possible grammar mistakes. Google Docs users will be able to right-click on underlined grammar mistakes to see suggested corrections.
The new grammar suggestions tool in Google Docs will initially be available only to those who apply for the Early Adopter Program.