Adding things to calendars is the biggest habit I've built from reading the book years ago. That's the one thing I recommend as the best takeaway if you only have one.
If you're just looking for a summary: there is very little point writing things in a list if you don't read the list, obvs.
The key to GTD is that if you actually do write everything in lists and calendars, then you are freed from having to remember things. If you can reach that inflection point, then it frees up a lot of mental capacity and is glorious. Remembering things is surprisingly mentally draining, as it basically consists of thinking about them every now and then.
Read the book.