This question admits that the societies in GATTACA and V for Vendetta are both dystopian societies; therefore, each society is going to share key components. The following items are generally components of any dystopian society.
- There is some kind of leader or concept that is supposed to be worshiped by the people in the story.
- Information, freedom, and independent thought are restricted or illegal.
- Propaganda is used to control people.
- Surveillance is used constantly to monitor and control the population.
- Uniformity is an important part of societal control. Individuality is not promoted.
- Society members are dehumanized.
Most of the above list holds true for both films, but some differences can be found as well. In both films, there is definitely something central that is worshiped. In V for Vendetta that concept/person is the tight government control and the Big Brother character named Sutler played by John Hurt. In GATTACA the worshiped concept is that genetic engineering can create a happier and more harmonious society. The second item on the list is true of both films. This is why V and Vincent are the hero characters. They dare to stand up and fight against the system. V's fight is quite public. He doesn't mind being seen doing his fight; however, Vincent absolutely does not want anybody knowing what he is doing. He must be Jerome at all times, and nobody can be suspicious of his actions.
The propaganda characteristic is probably a major difference between the two films. GATTACA's society has this dystopian component essentially absent. It's possible that it exists in that fictional world, but audiences are sold the idea that it isn't needed any more because the society has fully bought into the scientific model. V's world is filled with propaganda.
Surveillance is a major component of both societies as well, but it is different in its execution. V's society requires more physical policing by actual law enforcement officials. We see this in action quite early in the film when Evey is caught breaking curfew. Vincent's society is monitored through constant genetic tests. Vincent has to fake his blood samples just to be let into certain buildings or areas of buildings.
Individuality is definitely squashed in both societies. Every single person in GATTACA is supposed to have a specific genetically engineered DNA code, everybody wears black suits, and everybody acts like emotionless automatons. It's just what you do. V's society has the same aversion to individualism, but it is adhered to by people out of fear. Interestingly, the film's conclusion has people rebelling against the government while wearing the same mask.
Finally, both societies dehumanize. V's society sees people as tools that need to be controlled and used. Vincent's society has dehumanized people in a different way. A person doesn't matter. His or her double helix is all that matters. Each person has been reduced to a string of genetic base pairs.