Now first of all, I'll concentrate solely on book canon here, since you're asking about the ending of the actual novel series. In the games Geralt is very much still alive, however you construe that out of the book ending. In lack of an official English translation, let alone one I have at hand, the excerpts quoted here have been translated into English by me (as accurately as my abilities allowed me) from Erik Simon's official German translation of Lady of the Lake.
One thing you might want to consider is Ciri's vision. At the moment when Geralt is hit by the pitchfork, he remembers the time back in Kaer Morhen with young Ciri, when she repeatedly had strange visions and dreams, including about Geralt's death:
"Tell me again what she said", Vesemir commanded and emptied his cup in one gulp. "Word for word."
"Word for word doesn't work", Geralt said, his view directed into the blaze. "But the meaning, if there is meaning in looking for a meaning in there, was this: Me and Coën will die. Teeth will be our demise. Teeth will kill us both. Him two. Me three."
And if we remember, Coën indeed died at the battle of Brenna. When he is brought into Milo "Rusty" Vanderbeck's field hospital the following conversation occurs:
"Strange." Rusty tried to wipe his face with his elbow, but even that was full of blood. Iola came to his assistence.
"Interesting", the surgeon repeated and pointed at the patient. "Stung with a fork or some kind of partisan with two tips ... One prong of the weapon pierced the heart, there, please look. The chamber is doubtlessly penetrated, the aorta nealy severed ... And he breathed just one moment ago. Here on the table. Hit right in the heart he made it to this table ..."
"Do you want to say", the trooper from the Voluntary Light Cavalry asked gloomily, "that he is dead? That we carried him out of the battle in vain?"
"It is never in vain." Rusty did not lower his gaze. "But to say the truth, yes, he does not live anymore, sadly. Exitus. Bring him ... Hey, damn ... Take a look at this, girls."
Marti Sodergren, Shani and Iola bent over the corpse. Rusty pulled the eyelid of the dead man back. "Have you ever seen something like this?"
All three shrugged.
"Yes", all said in unison. They looked at each other, as if being a little astonished.
"Me too", Rusty said. "This is a witcher. A mutant. This would explain why he lived so long ... This was your combatant, you people? Or did you bring him by coincidence?"
"This was our companion, sir doctor", the second voluntary, a beanpole with bandaged head, confirmed grumpily. "From our squadron, a voluntary like we. Alas, he could handle a sword! His name was Coën."
So Coën did indeed die (and Rusty's competent medical opinion leaves no doubt in this) from some kind of two-pronged fork, like the vision prophecied. And Geralt is just about to be stabbed by a three-pronged pitchfork, too. This might already be a very strong hint that Geralt is indeed about to die right now.
And in fact Yennefer has a similar flashback (albeit not directly when she supposedly dies by trying to heal Geralt, but a little ealier before she helps Triss summon the hail) to an earlier experience of hers, when she unsuccessfully tried to kill herself and woke up in bed at the care of Tissaia de Vries, who also spoke about Yennefer's actual future death:
"You will live." The voice of Tissai was factual, serious, even strict. "Your time has not come yet. When it comes, you will think of this day."
What she indeed does right now.
Now it is true that those might just be red herrings to make us think the end is nigh when it actually isn't (and in fact this wasn't the exact point of Yennefer's supposed death yet). But seeing how both of them have flashbacks to some kind of death prophecies, one very clear and accurate and one a bit more fuzzy, it is a strong motif that adds to the bigger picture.
Then we have to consider the mythical nature of their waking up on some strange unknown meadow supposedly being together forever. Neither do I think this is just one of the many parallel worlds that Ciri can just switch through at will and where she dumped them to be happy for now and to visit them whenever she pleases, it is somewhere else, it is something else. This is evident from her tearing eye when she recounts how Geralt and Yennefer marry later, with all the other living and dead characters joining the party, an obvious sugar coat ending:
"So what was then?"
"Well, what", she snorted. "They married."
"Ah, what's to tell there? There was a happy feast. Everyone came together, Dandelion mother Nenneke, Iola and Eurneid, Yarpen Zigrin, Vesemir, Eskel ... Coën, Milva, Angoulême ... and my Mistle ... I was there myself, ate and drank. And they, to say Geralt and Yennefer, later had their own house and were happy, very, very happy. Like in the fairy tales. You understand?"
"Why are you crying, Lady of the Lake?"
"I'm not crying at all. My eyes tear from the wind. And that's it!"
So I don't think Ciri could ever reach them again, nor bring them back anywhere else.
From all those aspects and the general tone they set added together I would draw the conclusion that they are indeed dead, or at least not "alive in a meaningful way".
Wherever strange mythical paradise they are, be it heaven, be it Avalon, or whatever you want to call it, in their very universe and in their very reality they are for all intents and purposes dead, realistically speaking.
But I won't deny the fact that the ending is indeed a little ambiguous and open to interpretation. This is merely my conclusion based on how the story was presented.
As an addendum, the rather new standalone novel Season of Storms from 2013 does reference Geralt's possible fate and survival in its epilogue.
Where he saves the young Nimue from a monster on her way to Aretusa, 105 years after his supposed death.
However, that whole epilogue is pretty much as mysteriously ambiguous and open to interpretation as the ending of the novel series. It does not make a clear statement about Geralt's actual survival or what happened to him at all either, or if the described encounter even happened in reality. If anything it rather supports the mythical themes of afterlife invoked in Lady of the Lake.