In this case, the door opens into a library and admits what appear to be...

a pair of small unicorns with images of a shrub and a book on their flanks.

Inside, they can see a restaurant with a bar. There are stools at the bar, alcohol lining the shelves, copious seating- seating as far as the eye can see- and people. None of them look like ponies, or dragons, or anything Gloss or Gossamer would recognize. Some have scales, others feathers, but most are bipedal. They chatter, laughing and arguing at loud volumes.

There is a spiral staircase which seems to lead up and down, and there's music playing. There is no one attending the bar.

This is very surprising.

Gossamer will go ask people what's going on.

"This is Milliways, the only bar at the end of the universe! The rest of them have gone out of business, see. People come here from all sorts of places and time periods and what-have-yous. You're like the wildest mammal that I've never seen. How'd you manage on four legs, anyway?"

"It is curious, isn't it? Can a species really manage to evolve without reaching a bipedal state? I can scarcely credit-"

"Evolution can't be real in every universe, Erdiga. Perhaps this...horse...comes from a universe without it. If I were to venture a guess-"

The three of them continue arguing, forgetting the source of the argument. Their remaining dining companion exhales a noxious cloud, which seems to communicate quite clearly to Gossamer despite the lack of any vocalization:  "You should talk to the Bartender. They'll keep on like this for hours."

Well, she doesn't see a bartender but she can go wait for one at the bar. And also relay what she heard to her sister.

The bar might sense that, while she is capable of eating material food and drink, doing so gives her neither sustenance nor pleasure; she instead lives by magical parasitism.

If the bar senses any such thing, it does not say as much. The suddenly appearing Bartender, a portly gentleman with several tentacles extending from his back, says, "Welcome to Milliways! What can I get for you, madam?"

"I'm told I should ask you what's going on?"

"Ah, yes. A first time visitor, are you? Well done, well done. This is Milliways! We cater to patrons throughout every universe. No matter what point in time or point in space you come from, we welcome you. Oddly, some points of time and space seem the same yet contain different sorts of things. Most patrons talk about different universes or dimensions or worlds, although I'm afraid there's no truly consistent terminology that I'm aware of! So you could be from the same exact time, down to the minute, and in the same exact place on the planet, as another patron, but you might have completely different physics or magics! The drinks and food are priced with that in mind, of course."

"Wow!

Howcome the door out of the Golden Oak Library lead here?"

"When Milliways appears anywhere and anywhen, it appears as one might expect, behind a door! Any door which might normally lead to some other, more ordinary location, can instead lead to Milliways. It won't do so forever of course- or rather it will, for a certain sense of forever."

He winks. Additionally, two of his tentacles perform some kind of gesture. Perhaps this is body language for whichever kind of people this Bartender comes from.

"What sense of forever will it always lead here in?"

"Why, as long as you remain inside our establishment, it will lead back to your home. So, for you, it may last quite a long while. Quite a few people use Milliways to get important work done when they're running full-speed towards a deadline outside. Would you like something to imbibe?"

"So no matter how long we spend here, it'll be the same time when we go back?

"With a few exceptions, yes!"

"Hmm. Well, thanks for the information but I'm not thirsty at the moment."

She'll bring Gloss up to speed again then look for anyone who looks like they want to talk.

Quite a few patrons glance, but most are in conversations of their own. Only one stares in undisguised curiosity. He waves when he realizes he's been noticed.

Then one of the unicorns will approach him.

"Hi!"

"Oh, hello! I- you seem- my name is Satish."

He extends a hand and then retracts it, running it through his hair.

"I'm Rainbowstar. Are you new here? What's your dimension like?"

"I don't know how to describe it. It's different than here since I can't hear God, and no one really asks me to help them. I guess, relaxing?"

"What's God? What do animals ask you for help with at home?"

"You don't know what God is? That seems hard to miss...who kept civilization together through the early days? God created humans and kept human civilization growing to where it is today. They're the one who created advocate-scapegoats, like me. I help people by taking the blame for their sins."

"I think in my world different animals kept civilization together in different parts of the world. And what are sins?"

"I've never had to explain sins."

He thinks for a minute before answering.

"Sins are mistakes that damage society. Poor decisions made for stupid or selfish reasons which undermine the social fabric. Most people are allowed to sin a little, as long as they manage it well enough. Phliosopher-kings and noble-scholars can't sin, or the people would never be able to trust their leaders. It's a delicate balance."