Grindelwald's for ages when Voldemort rises to power. He still hears news from the guards and whatever papers he's allowed. At first he's amused. The Wizarding community didn't want him as a ruler so someone worse rises up to take his place. Then, people start talking about the animosity between Dumbledore and Voldemort. And then Grindelwald starts to get jealous. Who is this Voldemort guy and who does he think he is? Dumbledore's archenemy? That position's already been taken! Oh, they have a complicated history because Dumbledore was once Voldemort's teacher? Well Grindelwald was once Dumbledore's BFF/crush! Take that!
Eventually, Grindelwald decides that he's just going to need to break out of prison to put this Voldemort guy in his place.

Bonus if Grindelwald's thoughts get seriously childish. (Ex. What time of name is Voldemort? Rhymes with Moldy-more!)