Wikipedia doesn't provide the translation for "Gee Joon" in English, and on the Chinese version, it gives the terms 至尊宝，猴王對，皇帝，and 至尊天九, which don't seem to match the transliteration (https://zh.wikipedia.org/wiki/牌九). Does "Gee Joon" follow a Cantonese romanization scheme, or is it just a English approximation?
I believe that it is just the shortening of 至尊天九 or 至尊宝 to:
Wizard Of Odds mentions:
Supreme Creator (Gee Joon): Chinese name of the Supreme Creator, which came before anything else.
Gee Joon means Supreme Creator, which fits the characters 至尊.
Chinese Dominoes back this up pretty clearly:
This pair when played together is considered a suit on its own, called the Gee Joon (至尊 Supreme). It is the highest ranking pair in the game of Pai Gow, though the tiles rank low individually (in their normal order).