How was disillusionment experienced by characters in the Great Gatsby largely due to World War I, PTSD, and the influence of their past?

World War I hovers as a presence in this novel, though it is never stated that anyone suffers form PTSD or what then would have been called "shell shock." Gatsby does recognize Nick Carraway as someone who served in the armed services in France, as he himself did, in World War I, and that undoubtedly helps cement the bond between the two. World War I also separates Gatsby from Daisy: Gatsby comes home from war to realize she has married another man.
Gatsby is not so much disillusioned with the past as desperately trying to recapture it by reuniting with his first real love, Daisy. He believes he can erase the five years since they were last together and pick up where they left off. If Gatsby experiences any disillusion, it is because the Daisy of the present, when they finally reconnect, can't possibly live up to what Gatsby has conjured in his dreams.
The person in the novel who experiences the most profound disillusion is Nick. This has nothing to do with World War I (as far as we know) but with the "foul dust" he encounters in his summer on Long Island, particularly the foul dust of Tom and Daisy, who play with people, wreck their lives, and then use their vast wealth to run away from the mess they have made. Nick runs away at the end of the novel, back to what he pictures as the cleanliness and purity of the midwest, with its picturesque snows and Christmas wreathes.