It's the same as GIS, you wouldn't say jizz now would you?
Because the other assumption I could have gone with is the less charitable take that they know GIS with a soft G doesn't sound like jizz, but they were just looking for a crude way to mock the soft G.
Some men just want to watch the world burn. At least it's mostly harmless fun anyway. It's even funnier when they bring up how my name is pronounced in defense of "jiff" and I tell them, so you're calling me the expert in "Gi" pronunciation then? :)