You can push the ground electrode on something hard to close it. Don't push it hard, they bend easily.
To open it up I use a gapper tool Performance Tool Combination Tune-up Thickness Gauge : JC Whitney: Auto Parts & Accessories
. available at any auto parts store. I recommend the ones with a wire feeler gauges for checking the gap setting.
You can also grip the sides of the ground electrode with a small pair of vice grips. Do not clamp them down hard though, you do not want to crush pinch the little thing. Never, ever use a screwdriver to pry the electrode open, You will run a high chance of breaking the center elecrrode if you do.