It’s a bastardization of capicola. Unless they think it is somehow making fun of Italian Americans not pronouncing things then I don’t see it either.
From some article:
In the case of gabagool, it's a combination of end vowels being deleted, "oh" sounds being raised, and what linguists call "voiceless consonants," namely "k" and "c" sounds, being turned into "voiced" consonants, which, in this case, amounts to "g" sounds.
So, wanna get your Soprano on? Start with capicola. Drop off the end vowel. Turn the "c"s into "g"s. And emphasize the "o" sound. Whaddya get? Gabagool. Fuhgeddaboutit.