Actually if you wanna be technical it's pronounced like a gutteral "gaet-ler-haert" told to me from my anthropology department head at Tulane university. (I curiously asked) but yes, people are correct that in English we promise a GJ sound as "y'all" so there's really no wrong way to say it :) but the g is very much pronounced in the word's root language.
English
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I've been pronouncing it "guh-YALL-erhorn": hard G, silent j. I was pretty sure the j was silent (as in "mjolnir", I believe); everything else I pronounced as in English.