Err, I hope it's a little quicker than 58ns...
Also, I very much doubt that this scope is Gaussian response. I think it's one of the magical downshift-frequencies and reassemble things versions, which complicates everything. (DSA70604 - the book says that it should be 65ps 10%-90% and 45ps 20-80%. It's software upgradeable to the full 20GHz, so I'd expect that all I'm seeing is a DSP filter doing a brick-wall impression to get it down to a measly 6GHz...)
(I also have a few proper analogue scopes to hand that it'd be fun to test. (Analogue from 10MHz to 1GHz, and a few points inbetween).