Grammarly doesn't work with Google Docs. If you have the Grammarly extension on your browser, there is a little x over the icon. Is this due to some disagreement between the companies, or some competing product Google may have?
According Grammarly Support, it doesn't support at this point, but you can use their online app to check it. I think they will support in near future.
It can be considered difficult to rewrite the code that allows Grammarly to work on sites like Google Docs. When you “Inspect Element” on the Google Docs text area, you’ll find that it’s not actually a
<textarea> element. In fact, it’s multiple wrappers of divs with one that is triggered to listen on key events to allow input.
The issue that I believe Grammarly faces is that the engineers are unsure of the best solution to target these “fake” text areas.
I think TheBrenny is onto something there. There is a way to get access to the content itself and edit it but it's difficult and would have a pretty big delay. You can take a look at add-ons API for google docs and on the "Things you can't do" section, it somewhat says this. Google addons Documentation
Also for docs events, if you wanted to do it directly from there you can only read document open events. Google addons triggers
They could, however, import the document right into the Grammarly editor and edit from there. You would not be able to do the collaborative editing but you would be able to update the document in google drive periodically (every second.) I actually think that would work pretty well personally. Write the whole thing out in Grammarly and then do all the layout work in docs.
I'm not 100 percent sure on this but I think it's likely because Grammarly would have to get permission from Google to run on Google Docs which is a live-saving kind of word platform. If you're writing an email or a YouTube comment, that doesn't save live which is why Grammarly can run on it. That's my best guess.