https://thispersondoesnotexist.com/
The above generates photos of people that does not exists, so if they do not exists, am I allowed to use their photos in quest game creator?

If they are photos of real/actual people, then my guess is they still remain the property of the person whose image it is and/or the person who took the photo. Unless there is a disclaimer that says they are free to use.
(By no longer exists, I presume you mean they are deceased.)

Also different countries may treat this differently and have other limitations.

I do not really know if they are photos of real people, but I did some digging information about the website, from my poor understanding of english, I guess the output of the photo is a combination from fake photo determined from a random code plus a real photo.

I do not see any disclaimer that says they are free to use, so i guess they are probably not free to use.

By no longer exists, I means they do not exists in the first place, they are generated fake people photo.

As far as I can tell from various reports on it, the images are generated by an adversarial neural network.

The copyright in those images presumably rests with the team responsible for training the network; but there's no indication on the site who that is. And it may be something that would never be an issue in any case, because if the site is generating images in real time, there's a chance there's no way anyone could know where you got them from. It generates thousands or even millions of random profile pictures… what's the likelihood they're keeping a record of them?

Sad, but I think they took down the website.
Edit: The website is back up, they were probably doing some server maintenance.