Gattaca was taking a lot of license for the sake of the movie. A lot of diseases are multifactorial and while genes might play a role, they’re not the only factor and often not even the main factor. The movie ignored things like epigenetics, early life exposures, lifestyle, age, sex, and just plain variance that are all factors that can play into chronic diseases.

That being said, allowing parents to make decisions based on genetic testing isn’t itself eugenics. Eugenics is a top-level idea revolving around the idea of improving the quality of human genetics as a whole. And that requires an institutional judgement of what are good quality genes and bad quality genes, which necessitates us saying some people are better quality than others and opens up the door to racism/homophobia/transphobia/ableism/etc. Eugenics is always bad, while personal medical decisions based on genetics can be reasonable.