In practice, the Gayssot Act allows the French government to fine and imprison people for questioning the Holocaust. Historians questioning details such as the figure of 6 million killed, or the use of gas chambers in certain locations have come under fire because of this law.
Most arguments in favor of the law center on the assertion that historical revisionism, when applied to the holocaust, is motivated by anti-semitism and is tantamount to incitement of racial violence. Proponents believe that this law is a moral necessity more important than freedom of speech.
I see many contradictions. If it is correct to curtail freedom of speech in order to prevent social disharmony, wouldn’t it be more efficient for France to ban all depictions of the Islamic prophet Mohammed? After all, that has been at the root of more racial unrest and violence than holocaust denial has. If, as the law states, it should be illegal to deny or minimize crimes against humanity, shouldn’t it be illegal to deny the Armenian genocide? France did briefly have a law criminalizing Armenian genocide denial, but it was overturned after some pressure from Turkey.
In general, I don’t see the actual harm of holocaust denial as being serious enough that it needs government intervention. It’s more or less the harm of being offended: there’s no evidence that holocaust denial has caused any violence against Jewish populations. People deny the truth every day: there are the flat-earthists, those who believe the universe is 6000 years old or created in 7 days, people who thought humans co-existed with dinosaurs — the list goes on. Though their opinions offend me in their ignorance, they don’t do much actual harm. There are widely publicized beliefs which do result in actual harm, such as “vaccines are dangerous” — and it would make more sense for the government to ban these types of statements (because they result in actual harm and sometimes actual deaths) before banning holocaust denial.
In fact, I think such laws, when enacted to protect only very specific groups (though the Gayssot Act technically covers all crimes against humanity, has only been used to prosecute holocaust deniers) tend to increase racial disharmony. It’s an obvious question: why is the government giving this group special treatment? Why is freedom of speech more important than the Armenian genocide, but not as important as the holocaust? It looks like the French government privileges some groups over others.
For a more thorough treatment and similar conclusions, see this article from Humanity in Action.