I am relatively new to all this, but have been running my pi-hole for about 6 months now and have been happy with the results so far.
I am looking to block more domains from my network. Specifically online gaming sites like addictinggames.com and stickgames.com.
I have added these to a github and then loaded that into my pihiole using the GUI but the sites are not blocked. I believe I have a format or reference issue that prevents these from being added to the pihole.
If you can access the site and it is blocked by Pi-Hole, then either the browser has it in cache still, or there is another DNS server available to the browser or there is another URL it is loading.
Pi-Hole will only display a block page if you configure it do so, and then only if the page is an http page (won't work on https due to required certs for SSL).