gross understatement

[AliBadass]

From the book millionaire success habits by Dean Graziosi: Though I love all of John Paul DeJoria's story, I relish that early admonition of his high school teacher who told John Paul and his friend Michelle Gilliam they'd never amount to anything. Well, to say that they proved this teacher wrong is a gross understatement.

I would appreciate it if you rephrased the last sentence for me. It doesn't make sense to me; I mean, proving that teacher wrong shouldn't be a gross understatement (a bad statement), because now they are successful (why is it bad?)

 

[Barque]

Please look at the definition of "understatement".

An understatement isn't a "bad statement". As the WR dictionary says, it refers to saying something less forcefully than you normally would, or making something seem less important than it is.

A gross understatement is something that's very much an understatement.
gross (WR dictionary): flagrant and extreme; glaring: [before a noun] gross injustice.

The writer means that they didn't just prove the teacher wrong but that they proved the teacher very, or completely, wrong. Merely saying they proved the teacher wrong isn't really enough to describe the situation. That's why it's an understatement.