I hope that some stronger players have some insight or that their are some quotations from notable chess players that relate to the question.
If you look only at the number of people that are obviously intelligent and put a lot of effort into their chess, and then look at the tiny fraction that achieve the level of national master or Candidate master (never mind the higher levels of FIDE masgter, International Master and Grandmaster) then it would seem that the answer is no, not everyone can become a Grandmaster.
But this begs the question. We do not know what made the difference. Was some of that effort and intensity wasted in the wrong sorts of study? Is some mental skill present in those that succeed that could have been developed in anyone if we only knew how? Does luck play a part? [Being in the right place, finding the right coach, stumbling on the right books, scoring highly your first tournament]. Or was it genetic?
The Pólgar experiment tells us nothing. It does not disentangle genetics from environment. László Polgár would have done well to learn something about designing experiments before he started. Then he might have adopted a child or two and compared their chess development to the biological children so as to exclude the genetic influence. Even then the results would not have been conclusive: a handful of subjects do not produce a statistically significant result.
Achieving excellence in chess is no different than achieving excellence in anything. There might be a genetic component. Where you live and are raised makes a difference to your opportunities and the quality of your training. Hard work is mandatory (even if some prodigies would prefer you to believe that it all came to them easily without ever having to work hard - because that is a myth).
What is the answer to your question? It is simple. It is the answer to many questions. It is not one that is popular. It is the reason for interminable debates and arguments, for cherry picking and for much opinion dressed as fact.
WE DO NOT KNOW!
I knew a guy from high school, whom I would characterize as a "genius," who is (now in his 50s), barely a master, that is, about 2200. He was a "teen prodigy" that won our league in our city in those days, for his age bracket. He was also a chess "hobbyist," who devoted most of his non-study/non-professional time to chess.
Mark could probably have become a grandmaster if he had devoted his life to it. But most less-gifted people cannot do this, even if they "try."
Well, that's a rather tough question. My answer is probably no, not everyone can become a grandmaster. Devoting yourself to chess to that extent requires tremendous amounts of time and money, which most don't have. If you really aim that high you'd have to neglect education and finally you would never be so sure you'll become a grandmaster. I think only those with true love and devotion for chess can go that far, because people aren't robots, you can't just make everyone love chess and force them to go as far as becoming grandmasters, I believe it depends on the person. However, if you have the 3 factors, deep love for chess, money and time, I do believe that such people can and will reach grandmaster level if they try hard enough. :)
There are a number of factors that would impact this, not the least of which is a strong desire to excel. A good memory is very important nowadays with the mass of opening variations and computer analysis having solved all the various types of endgames which you'd have to learn. The willingness to fight and nerves of steel are also important as well as a mortal fear of losing. And even with desire, talent is still significant. I know that some people will say that it is mainly hard work that will get you to the top. I worked all my life at chess but could never even get to Expert, peaking at a rating of 1891 over a decade ago. There are obviously some people with greater aptitude for the game who will excel at it, all other things being equal. How else do you account for prodigies like Morphy and Capablanca? Chess was their "native tongue". Not everyone can be an Einstein no matter how hard he works. How about Fischer's supposed reincarnation, Josh Waitzkin? He never made it even with all his advantages. The short answer is that hardly anyone can become a Grandmaster no matter how hard he works without inherent ability, a photographic memory, the willingness to spend a large proportion of his waking hours studying and practicing chess, plus a fanatical desire to win. And even those are no guarantee.
There is really no definite answer on this.
To check this hypothesis, we would need to get a sufficient number of random children aged six and force them (against their own will and that of their parents if necessary) to devote themselves to chess under the same conditions (e.g. trainers, books). This would be clearly unethical. With the Polgar family you have obvious selection bias.
Another practical problem is that if you pick too many children, you made it harder for each individual one of them to become a grandmaster (as you have to be successful in competition).
I suspect that a high percentage (but not all) of the children sufficiently pressured would make it, considering how easily children learn new languages at that time. But that is a personal feeling.
Personally, I find the Polgars still instructive. I regard them as weak evidence that the male/female differential is mostly cultural, which may indicate that the same could be true other strength differentials in chess.
What I have seen mostly is that people with 2400+ ratings had a very good genetics i.e. atleast one of their parents played Chess before or intelligent in their field. This absolutely boosts their children's Chess skills.
It is also dependent on your memorizing skills and having a clear mind. That's why Chess players indulge in physical activities boosting their body which ultimately leads to healthy mind. Vishwanath Anand likes to go to Gym in between Chess rounds. They cannot just devote their life for Chess and succeed.
It depends on what do you mean by "almost anyone". I think that any intelligent, focused, and with no other faults, individual, could make it if "they devoted their whole life to chess from age 6", especially if they could afford the best trainers and so on and on. GM is just a title, it means that you are a very strong player. That doesn't mean you are going to beat Carlsen or become US champion.