How does Chapter 1 of The Great Gatsby present the downfall of Jay Gatsby as inevitable?
At the beginning of Chapter 1, Nick relates that his father once gave him advice to remember the advantages he's had before criticizing other people. As a result, Nick says he is "inclined to reserve all judgments." Nick establishes himself as an objective person, a reliable narrator. We can trust what he says and that includes the hints he gives us about Gatsby.
Upon returning East, Nick didn't want to get emotionally involved with anyone, or as he says, he didn't want any "privileged glimpses into the human heart." However, he does get to know Gatsby (and Gatz). He adds that Gatsby, for all of his faults, was deeply sensitive. He was full of hope, idealism, and romanticism. And even though Nick has/had some scorn for what Gatsby superficially represented, he says that Gatsby was all right in the end and that he has never found another person with the same romantic idealism. Nick clarifies the "scorn," saying that Gatsby wasn't the source of his own ruin:
. . . it is what preyed on Gatsby, what foul dust floated in the wake of his dreams that temporarily closed out my interest in the abortive sorrows and short-winded elations of men.
The verb "preyed" is significant here. Nick indicates that something was out to destroy Gatsby: a predator. As we read further into the novel, we can consider some of those predators. Tom is a predator of this type. But more abstractly, the social structures that prevented Gatsby from attaining his dream are also symbolic predators. The selfish, superficial world of Tom's and Daisy's social circle preyed on Gatsby. The corrupt world of Wolfshiem is another.