There is no gasket between the down pipe (or the up pipe) and the turbo on a banks turbo kit. The oil feed line all you need are two 1/8" NPT x -4 AN right angle fittings and about 24" of braided oil line with -4 AN fittings from Summit Racing or a similar shop. I'm assuming your used kit came with the aluminum block bolted to the top side of the turbocharger, it should have two tapped oil ports on it, one is for that right angle fitting for the oil feed, and the other is for the oil pressure sender.
I'm referring to the Wastegated Sidewinder kit, I've never seen the older Banks kit, so I could be way off base if that is what you have.