In the language of Polandball comics, it means “give land/territory”.
“Gib” is taken from the German imperative for “give!”
Best theory is that “clay” as “land” originates from a bad machine translation.
It seems somene created a Wiki that covers these terms: https://www.polandballwiki.com/wiki/Terminology#Clay
After that project was started, the author started working for Brave.
The data and tests seem good, but some aspects of the methodology are opinionated. For example, browsers are tested in their out of the box configuration, not in a configuration that a reasonably privacy-conscious user would select with a couple of clicks. Thus, a browser dedicated to blocking tracking (like Brave) gets a lot more checkmarks than a general audience browser like Firefox.
LibreWolf is essentially Firefox with all those privacy features pre-enabled.