The manifold to Y-pipe doesn't use a gasket, it uses a male/female formed junction that compresses the thinner Y-pipe tubing over the thicker, stronger cast iron exhaust manifold, and if the surfaces are smooth and the bolts are torqued to spec, they should seal pretty well, with time and some rust or gunk sealing up the rest.
Usually it's best not to touch the sealing surfaces, but if they are no longer smooth, sometimes scraping them off or even using some steel wool, a wire brush, or aluminum oxide sandpaper can improve the surface to allow a better seal.
If the surface is too clean, as in there's nothing sealing it, sometimes using a thin film of anti-seize on the surfaces that make contact can do the trick. Anti-seize gets everywhere, so it can be a good idea to put it on only once the Y-pipe is in position and ready to be pushed into the manifolds, just a few inches below them, then push it on and bolt it up. You only need a super-thin film on the surface, otherwise it will squish out and make a mess.
Also, sometimes the bolts get the threads messed up with age, and the threading surface in the manifolds gets rusty and crusty too. Using a tap and wire brush (a circular wire brush for plumbing, or a battery terminal brush works well) to clean up the threads, and either using new bolts (available from the dealer) or making sure to wire brush the threads of the old bolts, (and maybe use a tiny bit of anti-seize on them as well) can make it a lot easier to get them torqued down to spec so the exhaust seals tight.
Sometimes you just need to use the right tool - a torque wrench will let you get the right amount of torque on the fasteners, where a short ratchet or even a small breaker bar will not.
