If you have read the books, yes, it would take some or whoever played the character would not ring true to fans or viewers (much like Tom Cruise didn't, although for a completely different reason). Reacher is not a one-dimensional Hulk like character that could be played by any dumb jock or athlete who is 6'5'' and has the physical build. He is intelligent, analytical and quietly conveys an intimidating presence or a menacing undercurrent without being a bully.
The photos I always see of Gronk off the field, he looks like a partying frat boy. There is a certain amount of gravitas that would be required of anyone undertaking the Reacher role and I kind of doubt he would meet that threshold. I did read that Gronk wants to pursue an acting career when his playing days are over, but based on this fake trailer he did for "A Gronking to Remember" he has, um, quite a ways to go:
https://www.sbnation.com/lookit/2015/7/15/8970547/rob-gronkowski-a-gronking-to-remember-patriots-nfl-funny-or-die-trailer