The reporter was told that he was not allowed to go into the room ... he barged in, anyway. That is trespass.
The reporter shoves a microphone in Gianforte's face in spite of being told to get out of the room.
I don't know if Gianforte's reaction was over-the-top or not, but this reporter absolutely deserved to be bitch-slapped. And, let's honest ... the use of the term "body-slammed" is nothing short of hysterical hyperbole. This is a body-slam ...

... and we all know that that is NOT what literally happened.
Gianforte might be guilty of misdemeanor assault, but the reporter is also guilty of at least criminal trespass. I would vote for Gianforte in a heartbeat, and I think the majority of Montanans will elect him as their representative in spite of this incident.

The essential American soul is hard, isolate, stoic, and a killer. It has never yet melted. ~ D.H. Lawrence