To unlock the Go Kick Rocks trophy in Days Gone, you will have to kick over 12 Anarchist Cairns.
There are a lot more than 12 in the game, and kicking them over has no reward other than unlocking this trophy, so once you have found 12 you can forget about them.
To knock over an Anarchist Cairn, simply walk over to it and press
.
You can find a video below which shows the location of 12 Anarchist Cairns (credit to Powerpyx on YouTube):