How did Grindelwald know about Voldemort?

Question

How did Grindelwald recognise or know Voldemort, since Voldemort was only active after he was imprisoned? He would surely never have seen Voldemort.

So, you have come. I thought you would… one day. But your journey was pointless. I never had it…

^{Deathly Hallows, chapter 23, Malfoy Manor}

Answer 1

No canon answer I'm aware of, but 4 likely possibilities:

1. Generally, in Muggle prisons, prisoners aren't refused newspapers and such.

   Even Sirius got a newspaper from Fudge in Azkaban:

   Black put one of his claw-like hands inside his robes and took out a crumpled piece of paper, which he smoothed flat, and held out to show the others.
   It was the photograph of Ron and his family that had appeared in the Daily Prophet the previous summer, and there, on Ron's shoulder, was Scabbers.
   'How did you get this?' Lupin asked Black, thunderstruck.
   'Fudge,' said Black. 'When he came to inspect Azkaban last year, he gave me his paper. And there was Peter, on the front page & on this boy's shoulder... I knew him at once... how many times had I seen him transform? And the caption said the boy would be going back to Hogwarts... to where Harry was...'

   So it's entirely feasible that Grindewald knew of Voldemort from Wizard media.

2. Prison guards talk.

3. Voldemort boasted of himself during conversation parts we didn't hear, as @madfriend's comment says. Plausible but less likely than #1 or #2.

4. Grindewald heard of Voldemort during his career as Dark revolutionary. He was defeated in 1945; Voldemort was already a brilliant student at Hogwarts between 1937 and 1944. He was just as brilliant as Dumbledore in his time; and Dumbledore was known academically by the time he graduated.

Answer 2

It's been quite a while since this question was asked but after the release of the movie "Fantastic Beasts and where to find them" I read an article in which J.K Rowling confirms that Grindelwald was a Seer.

So that could be a reason as to how he already knew Voldemort. It would also explain his short and cryptic conversation with Voldemort slightly better, like when he says "“You will not win, you cannot win! That wand will never, ever be yours—“". He already knew about the outcome of Harry and Voldemort's ultimate showdown.

The tweet from J.K where she confirms Grindelwald was a seer:

He is a Seer AND he was lying.

_{J.K.Rowling on Twitter, 11:48 PM - 18 Dec 2016}