No doubt Gronk is a HR swing, but we play fantasy football to win. If you can get him 5th-6th, then it's a good gamble IMO. There's no doubt in my mind that at that point in the draft, he is one of the only true difference makers left. I think if you look at the RBs going in the 3rd-5th, you are probably taking on similar risk in that they don't have any real guarantee of playing time, success, or staying healthy.
Peyton Manning was going at right around the same ADP last year (early- to mid-4th) as Gronk is this year. All of the questions that have been asked about Gronk's ability to bounce back, ongoing effectiveness, possibility for re-injury, etc. were being asked about Manning last year, in fact more so.
In the end, owners had to be willing to roll the dice with a 4th-round pick on the (clearly uncertain) possibility that Peyton would bounce back to something close to, well, Peyton.
Peyton won a lot of those owners championships last year.