The OED gives four pronunciations for British English but only one for American.
GEE-zuh, GUY-zuh, GAY-suh, GUY-suh.
American: GUY-zur.
Apparently the word comes from Icelandic, a former proper name. These days they call it a goshver, pronounced goss-kvair. Gos means an eruption and also a soft fizzy drink. Hver means a hot spring.