The issue has been resolved. Cloudflare posted a blog article concerning the incident: https://blog.cloudflare.com/cloudflare-outage-on-july-17-2020/

Situation seems back to normal on Cloudflare side. We are monitoring everything closely.

You can follow Cloudflare updates at https://www.cloudflarestatus.com/incidents/b888fyhbygb8

Our DNS provider (Cloudflare) is currently experiencing an outage which may affect your ability to access GitBook at this time (public content and dashboard).