I am trying to find the documentation for GUIs in 1.16.x but when I go to the link:
https://mcforge.readthedocs.io/en/1.16.x/blocks/interaction/GUIs
It glitches out / does what seems the be a broken-ish error 404 screen
https://mcforge.readthedocs.io/en/1.16.x/blocks/interaction/
I click on the link from this page.
Edit:
At the bottom of that page it does infact say "page not found"