I have honestly never ever seen a gate glitch like that. But what an interesting glitch if it was lol I've looked all over the forums and I can't find any cases of this happening. The repair hammer missing is an old glitch but this one allows you to just run through!
Yeah, it was reported before, although I imagine it is not easy to find the thread. Glitch occurs rarely, from what I see on the forum, but not 1st time. I imagine the gate was rebuilt after taken down, and sometimes rebuild goes wrong and you get a gate that is not closed. Not a hack, just a glitch.
There might be some factor or spell that affects the gate as it goes up and perhaps opponent managed to trigger it at the time gate was re-birthing. But I doubt that was deliberate even if it is affected by the client events and not just a server glitch.