You don't get it.
in the RAWs, it is written as GEASS.
If you were to write GEASS in katakana, as if it were a foreign word, then it would be "Ge-Ah-Su".
Basically, it's like taking a word, putting it in google translator from language X to Y, and then putting that word from Y to X.
Sometimes, it comes out a BIT differently.