In reply to Rasmus Jürs.

According to Google, there's 644,000,000+ websites. However, there's suggestions that roughly 200,000,000 are active.

Playing with the active number and going off WordPress's claim that it runs 20% of the internet leaves us 40,000,000 websites. If 450,000 of those installed Gutenberg, that's about 1.125% of all WordPress sites running it.

All approximates of course. We have to assume that some unknown number of WordPress websites would count as inactive; I'm just giving it the best chance possible in my admittedly flawed math.