From what I'm hearing from various other sources is that this was actually a shove to the guy's neck area after he pushed his way into a private office and stuck a camera (or recording device) of some sort into Gianforte's face.
And no, I can't confirm the source of that. It's a brief summary from a quoted article that I read on another forum, which I don't have in front of me at the moment.
But, if that's true, then it wouldn't surprise me. Because, of course, to the media, shoving a pushy, trespassing reporter equals "bodyslamming an innocent journalist".