I thought it looked a bit like an Old English word maybe resurrected for D&D, so I initially thought something like /gεɑs/ (a bit like "gas" or "GEH-ahs"; ain't no player actually gonna say /ɣ/ or /æɑ/ properly) or /jεɑs/ ("yasss")

Then I looked it up on Wiktionary. It's from Irish "geis" with the wrong spelling apparently. Irish spelling do be silly, so all phonetic preconceptions should be checked at the door.

Wiktionary says /ɟɛʃ/ for Irish, anglicized as /ɡɛʃ/ or /ˈɡiː.əʃ/ (gesh and GEE-ush, respectively).