You’ll find this Checkpoint in the end of the main road passing through the Old Pioneer Cemetery (you’ll get to this cemetery as part of a Story Mission). Once you reach the checkpoint, climb on top of the guard booth to disable the first speaker then push the SUV out of the tent’s entrance.
Enter the tent and head inside the Checkpoint’s compound. There are freakers inside, with the gas can located near their feeding ground. Take them out using any method you want, just don’t shoot the gas can.
From there, it’s very much straightforward. Use the crates beside the MMU to get to the roof, disable the speakers there, Refuel the generator, and start it up to get inside.