I'm not a phonetician1, so this is going to be a very amateurish answer from a phonetics perspective.
Different characters on the show typically used one of two pronunciations2:
go-aah-oold, typically used by the System Lords, Tok'ra, Jaffa, and nerds like Daniel Jackson
goo-ld, typically used by other military characters, like Jack O'Neill and General Hammond
Just based on who typically uses the different pronunciations, I'm inclined to suggest that go-aah-oold is the correct one; always trust a people when they use their own name for themselves.
Dan Castellaneta's character, the eponymous "Citizen Joe", also feels quite strongly about the pronunciation:
Wikipedia user Puellanivis suggests the proper IPA rendering as [ˈgoɑ.ʔuːld]:
I'll readily agree with anyone who says that the common pronunciations of "GOOLD", and "go-OOLD" is mispronouncing the word, but the pronunciation listed in IPA: [ˈgoʊ˘uːld] is not entirely accurate either. From the correct pronunciations that I have heard, the ' in fact stands for a glottal stop, which would make it consistent with Arabic transliterations. (How one would resolve this with the name "Teal'c", you got me, my best guess is that who ever was making up the names, wasn't a linguist.)
From what I hear, the most accurate pronunciation would be: [ˈgoɑ.ʔuːld].
1 Hey, I learned a new word!
2 With some allowances for regional accents. Jack, for example, is canonically from Illinois3 and played by a Minnesotan, usually pronounces it goold. Compare to General Hammond, canonically a Texan and played by a Missourian, who usually says gewld.
3 "The Fifth Man":
O'Neill: Where're you from, Tyler? Texas
Lt. Tyler: Mostly. We moved around a lot. You?
O'Neill: Born in Chicago, raised in Minnesota.
Stargate SG-1 Season 5 Episode 4: "The Fifth Man"