gpurender.py

import bpy

bpy.context.user_preferences.system.compute_device_type = 'CUDA'
bpy.context.user_preferences.system.compute_device = 'CUDA_0'

bpy.context.scene.cycles.device = 'GPU'
print(bpy.context.scene.cycles.device)

render.sh

#!/bin/bash

# it's super important to give the script AFTER the Blender file so that the scene can be set to GPU.

blender -noaudio --background file.blend \
  --python gpurender.py \
  --render-output /tmp/ --render-anim --frame-start 1 --frame-end 140 \
  --engine CYCLES