I was actually just kind of musing on how the 'attack' was initially spun, not making any kind of personal characterization.
That being said, if you want to talk about professionalism, the reporter was first denied comment and then referred to a spokesperson. At that point, he should have put down his recorder and stepped aside. There was no scrum - and the question was not so indisputably newsworthy that it required cornering and repeatedly badgering Gianforte. There is no defending what the (now) Congressman did in response, which, make no bones about it, was a gross overreaction, but let's not pretend that the reporter's behavior was entirely above board either.