It's a milestone case where a serial rape victim will testify after a multi-decade wait for justice.
Former interior minister Ousman Sonko served under ousted dictator Yahya Jammeh.
He becomes the highest-ranking official to be tried in Europe under the principle of universal jurisdiction, which allows grave crimes to be prosecuted anywhere.
That's according to the Swiss campaign group TRIAL International, which filed the complaint against him.
Nine Gambian plaintiffs are coming to Switzerland for the hearing.
Madi Ceesay is now 67 years old. He alleges he was falsely imprisoned and tortured under Sonko.
"I am smelling justice. And I'm anxious to come to Switzerland and watch the proceedings and see what the final verdict is going to be. But I'm so optimistic that justice will be done. And I always say, even it stops where it is now, for the past six years, he's pinned down at one place. That alone is I think a kind of relief for me."
Sonko was arrested in early 2017 while seeking asylum.
He faces charges including murder, multiple rapes, and torture between 2000-2016 in what is Switzerland's second trial ever for crimes against humanity.
He denies the charges. But if convicted, he could face the maximum penalty of a life sentence.
Sonko's lawyer, Philippe Currat, asked the court to abandon the case, citing problems with the investigations and hearings.
He is expected to argue that many of the alleged crimes against humanity happened before a relevant Swiss law took effect in 2011 and are not admissible.
Former President Jammeh's 22-year repressive rule ended in January 2017 after he lost an election and was forced to flee.
Fatoumatta Sandeng is eager to see the trial unfold. Her father was an opposition activist who was killed in custody in 2016.
"We are hoping that we could get Jammeh one day, because that is the ultimate goal ... If we don't hold people accountable, things like this will keep happening in Gambia, in Africa, all over the world."
