Who are the original inhabitants of the world of the Witcher 3?

It would indeed appear that gnomes were the first (known) inhabitants of the world The Witcher is set on:

“Elves!” snorted Yarpen. “They—to be accurate—happen to be strangers just as much as you humans, although they arrived in their white ships a good thousand years before you. Now they’re competing with each other to offer us friendship, suddenly we’re all brothers, now they’re grinning and saying: ‘we, kinsmen’, ‘we, the Elder Races’. But before, shi— Hm, hm . . . Before, their arrows used to whistle past our ears when we—”

“So the first on earth were dwarves?”

“Gnomes, to be honest. As far as this part of the world is concerned—because the world is unimaginably huge, Ciri.”
_{Blood of Elves, chapter 4.}

Yarpen says "white ships", possibly as a reference to The Lord of the Rings (of which Sapkowski is an admirer), but it doesn't contradict with the fact that elves came from another world, too - after all, the Wild Hunt used a ship (Naglfar) to travel between worlds as well.

In a later book, an elf who is the closest thing to authority on this subject we have, says elves used to travel between the worlds freely, and that the elves who chose to stay on this world are called Aen Seidhe, while the elves who live on another world are called Aen Elle:

“We, Aen Elle, we cared very little about the deeds of your ancestors, we unlike the Aen Seidhe, our cousins, we left that world a long time ago. We chose another universe, more interesting. In that time, it will surprise you, it was possible to move freely from one world to another quite easily.”
_{Lady of the Lake, chapter 5}

So elves came by their own will, back when they were able to travel between worlds as easily as Ciri can, to this world and tried to impose their dominance on the dwarves and gnomes.

The Conjunction happened afterwards, and that's when the humans were introduced, after having destroyed their home world:

"And then, suddenly, comes the Conjunction of the Spheres and the appearance of humans. The remnants of humanity fleeing from another world, your own world that you totally destroyed, with your own hands that were still covered in hair, just five million years after you formed as a species."
_{The Tower of the Swallow, chapter 7.}

Other creatures were introduced at the same time, if we are to believe the words of a certain vampire:

"I have lived on this earth for four hundred and twenty-eight years according to your reckoning, or six hundred and forty-two years by the elven calendar. I’m the descendant of survivors, unfortunate beings imprisoned here after the cataclysm you call the Conjunction of the Spheres. I’m regarded, to put it mildly, as a monster. As a blood-sucking fiend."
_{Baptism of Fire, chapter 5.}

I think it's wrong to say that magic was introduced to this world with the conjunction - it's better to assume humans were able to tame magic after arriving to this world with the Conjunction, as per Yennefer's explanation:

"Magic is, therefore, the revenge and the weapon of Chaos. The fact that, following the Conjunction of the Spheres, people have learned to use magic, is the curse and undoing of the world."

[. . .]

"The fact that, following the Conjunction of the Spheres, some chosen few discovered talent and magic within themselves, the fact that they found Art within themselves, is the blessing of beauty."
_{Blood of Elves, chapter 7.}

We can join the following facts together:

• Humans destroyed their own world

• Humans are said to have been primitive when they first arrived

• Humans made a great deal of progress after being introduced to magic

We can speculate that there was no magic on humans' previous world, and that may have led to them destroying it (how? this I can't answer) because they were so primitive. It's thus possible that magic is somehow connected with sentience, but this theory needs a bit more development.

We know about first inhabitants of witcher world from Andrzej Sapkowski's books - there is a conversation between Ciri and Yarpen Zigrin (dwarf) about history of people in this world and about first races.

So first there were gnomes. Next arrived dwarves and haflings. Next arrived elves and conquered all races thanks to magic (this is a reason why dwarves are inhabitants of mountains - all good areas were conquered by elves). The last race on continet were humans. They arrived in small number and weren't destroyed by elves because women of elves recognized human males as good sexual partners - they could have more childrens with humans than with elves. Humans quickly reproduced, they also could use magic, and after several houndreds years they conquered elves.