Try moving to different parts of the fence.
I have ran all along the fence, from one end to the other, I actually even tried throwing grenades (both by hand and with the grenade mg) at the guy to see if he'd move and he just stood there.
Also, If you head more to the east from the main gate almost to the railroad tracks, theres a rock that almost overhangs the fence, I tried to jump it and there's an invisible wall above the damn fence.
My strategy was just keep tucked up to the rocks on the left and run like hell.
Edit: alright, I started a whole new game on hardcoe mode, and the one of the first things I did was test this glitch and the guy sprinted straight for me once I reached the gate. This glitch svcks but atleast I can still beat the game.