Of course this could be a mistake but Coppola supposedly hired guys who, if were not in the mafia, were at least from the old neighbourhoods in New York and would have known that gabagool and capocol were the same thing. The interesting explanation I heard (and sorry I can’t remember from where) is that capocolla has many variations from all around Italy, each with a different flavour depending on its region of origin. The variety favoured by Italian immigrants (mostly from the south) was often spicy and when they went to America, they had to make it themselves. Given the ingredients of America, it was different still to any of the varieties back in Italy. Perhaps, old school Italian-Americans (especially immigrants) distinguished the types made in America and those in Italy by giving the names gabagool and capocol respectively. If this is true, it would have been out of date by the late 20th century when less attentive Italian-Americans saw them as more or less the same (think how an Italian makes carbonara vs how an Italian-American makes carbonara—completely different). If he was talking about only 3 sandwiches versus 4, it still sounds strange to me. It’s like saying a ham, ham sandwich. So, I like the idea that he could be saying something more akin to 2 ham and a prosciutto (2 different styles of the same cut of meat). But again, maybe, just maybe.

According to Wikipedia they are the same thing.
https://en.m.wikipedia.org/wiki/Capocollo
Here is the scene:
https://getyarn.io/yarn-clip/e5413e90-8a6d-4e85-91ab-2d10a40fedef
He says he has "two gabagool, capocol, and a proscuit," which would be four items, but he only throws out 3. He must have been refering to the gabagool and the capocol together.