Originally posted by Zep Tepi:

Whichever camp you are in when the timer goes off and the dice roll happens.

A camp has three special items within a certain radius of each other, usually fire, workbench and bed. It doesn't care which one, could be three fires.

One time I actually used a gate just as they attacked. They attacked the building I left from and where I think my last bed. I didn't see the attack warning prior to leaving but saw it warn me as soon as I arrived (confirmed by opening the map and saw the red circle). So I had to gate back. This was from the previous version to the latest update from the date of this post.