Gefilte fish is usually carp, white fish, or pike. Freshwater fish and way different texture than salt cod. I agree if you want to use salt cod make a brandade de morue, or onoe of the recipes for bacalao.
I just worried the texture might be off. I don't have to use it but happened to have some in the fridge so it crossed my mind. Probably just go with the usual types of fish and leave the experimentation for another day when a dozen people aren't coming over.