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That would be my next step. Block it so that they can see through but not get through the doorway. See how that goes.
Then you can try to open it and see what happens. Usually if there is a conflict, each party will respect territorial boundaries until things improve.
Even if something does go "wrong", all is not lost. They can forgive and forget. Every day is a new day for a cat. It just might take some more time.
When you can get them in the same room, playing with both at the same time is good, like with a wand toy. Each can be focused on the play and sneak a look at the other at the same time. Hard to be afraid of a playing cat.
Hissing and even growling is just cat politics as usual. Just ignore it. It will go away eventually. You will know you are "there" when they don't hiss or growl but just sniff each other.
If the older one is annoyed by the younger you may still get a hiss or growl that says, "back off and leave me alone!" even after several months or years. Not to worry as long as no fur is flying. 
I think probably its been near 26 home cat to newly arrived cat introductions here at my home. All but 2 were successful. After I had the newly arrived cat worked up for any of the usual parasites and obvious infections we opened the closed isolation room. Had a homemade door frame with chicken wire placed within the existing door frame. Here the face to face introductions began. At first with my supervision with a water spray bottle in hand. Day or two later I could safely walk away and let the home cats satisfy their curiosity about the new kid in the iso room. The wire screen door frame would be kept in place anywhere from 5 days to 2-3 weeks. Odd thing is that when the wire frame was removed the home cats would mostly ignore the new kid and would be more intent on investigating in great detail the contents of the iso room. Meanwhile he new kid would be mostly free to explore far and wide their new home.
Photo of my homemade door frame here: <img src="
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Kitteh conference. 
Maggie's the one that usually starts it too.
And they're often pretty close to each other when it comes to taking naps.
I kept the gate up in the den doorway for weeks, fed the 2 cats simultaneously a few times, shared their scents, and the cats seemed to be able to handle closer and closer proximities to each other, so
now the gate is open (gate has been moved to the hall door to the bedroom where the original "home" cat, Krishna sleeps with us on the bed, while restricting the new cat, Opie, to the living room/den/kitchen/dining room area.).
When they 1st met face-to-face without a gate between them (a couple days ago), Krishna ran away,
but was later approached by Opie as she sat in a sheltered corner of the room, where he licked her forehead).
Things looked promising at that point, I thought. He really seemed to like her.
But now it seems, Opie's more dominant (or younger, curious and more playful?) male personality seems to be emerging.
She (Krishna) likes her space, and he is invading it, constantly ----Standing between her
and her food-dish (he's got his own in the den), making her back-off and walk away instead of eating,
and always wanting to be near her when she wants SPACE.
He is always wanting to be around her (probably playfully, it would seem), but he is making her
uncomfortable in her own house, as Krishna can't seem to go anywhere without Opie following,
or crowding her. I am willing to see this go on for a while, but will it ever change?
If not, I will be having 2nd-thoughts about keeping Mr. Opie.
And that hurts.
As long as I am there to make sure she can move-around freely without Opie crowding-in, and keep him out of the bedroom/hall area, and letting only HER go outdoors, then she has places she can flee to where she can have peace.
I am not letting Opie outdoors or in the hall/bedrooms yet (places where Krishna goes freely).
These are turning into her refuges, sadly. She mainly just runs from him now.
She hisses at him when he gets too close, but he doesn't back off much.
But he seems excited and "buzzed" to be free to roam a great deal of the house at this point.
The problem is he is not leaving Krishna alone, and she doesn't seem able to "make her space" with him, at least yet.
When she is outside, he lounges relaxedly on the couch, but usually follows her when she comes in.
I am at the end of what I can do, I believe.
I'm willing to give it more time, but it hurts me to see the sweet, aging Krishna made
afraid and intimidated in her own home.
I'm starting to think about the possibility of taking Opie back to Good Friends Adoption Center,
and it makes me feel like a stupid, miserable failure.
Why can't we all just get along?
And this whole affair has made my sense of loss from last December's devastating (for me) passing
of our wonderful boy, the gentleman Ragdoll cat Mukluk, even harder to live with.
I am so sorry to everyone this is not a more uplifting post, but the story isn't over yet,
so maybe there is yet some small hope ...?
I will try to be patient, but if things don't get better ... ?
