I generally don't use any tool. No grammarly(not even sure what it is...) no autocorrection, no word.
I'd love to say that I do because I feel very confident about my writing skill, but to be honest the truth is I just never took the habit.
A while ago, I used to write everything in drafts on word so there I had the thingy that marks with red your typos, but eventually I started loosing tracks of diffrent version I had here an there and got back to just write directly here.
Lately I do that 90% of the time.
That said, my opinion may feel biased on this, but I think it's best to write 'free' if you care enough to try to improve. After all, I did learn to write in my mother language that way.
Also, I'm a little afraid that a tool that checks the readability of my writing suggesting a better structure for my sentences (read more common) could hider any impulse or passion or personality or whatever you want to call it that I feel it needs to be put in it, even if it's just third grade smut.
Anyways, who knows, maybe one day I'll give it a try and won't be able do without anymore.