I always pronounced it as if to almost rhyme with Delorean. The first time I heard otherwise was some years ago when Matt Lucas was on some show claiming he could identify any vintage Star Wars figure blindfolded. When passed the gammy, he called it Gammo ( rhymes with Zammo) - Reean ( rhymes with wee 'un).
A Gammorrean in a Delorean? Or Zammo Wee Un? I go for the former.