https://gajim.org/downloads.php?lang=en#ubuntu

Has

wget -q https://gajim.org/gajim.key -O - | apt-key add -

Should be

wget -q https://gajim.org/gajim.key -O - | sudo apt-key add -

Also all instructions start with a whitespace when copied into the clipboard, which on some shells will exclude them from history, so users may not be easily be able to see what they did when they face issues. The whitespace should be removed from the HTML.