This has been bothering me since I saw the first trailer, so I a little research. It seems to be based off an Celtic/Gaelic word meaning a binding contract. Check out the wiki page for "geis" for more information. One site says:
"In Scots Gaelic it is spelled "geas" and there have also been other spellings. Pronouncing it is an even more tricky problem. My Irish books tell me that it's said as "gaysh", but the Oxford English Dictionary gives three different pronunciations, including "gesh" and "geesh" as well as "gaysh", of the three preferring "gesh". The Shorter Oxford gives this as its sole pronunciation. The one-volume Oxford Dictionary of English weirdly prefers "gas". No other dictionary I have here includes it."