How to ensure that eggs get hard boiled on a gas stove?

I agree with @jwenting, 5-7 minutes in boiling water is the way to go. Remember to always put the eggs in cold water, if you put them in hot water you risk cracking the shell, especially if eggs are cold.

Also, to avoid premature cracking of the shell, that may leave some egg white leaking out, add a spoon of vinegar (should work with lemon juice too) to the water.

Just watch the clock, it's the most (if not only) reliable way. And make sure the water stays at or just under boil of course. There's no difference between cooking them on gas or electric, it's just a different way of heating the water :)
Depending on how firm you want them, 5-7 minutes should usually be enough in boiling water.

I really like using the egg-perfect egg timer. You boil it with the eggs and it tells you when the egg is finished. It actually tells you whether the egg is soft, medium or hard boiled.

I always use Julia’s method. Prick the eggs on the rounded end (a tack works well), cover with water, add a splash of vinegar, bring to a boil uncovered, immediately remove from heat and cover, after 17 minutes immerse in an ice water bath for at least 2 minutes. Also, I’ve found that the fresher the egg, the harder it is to peel, I like to use eggs that I’ve had a couple weeks.

I always pierce eggs at the broad end but I have found that I can make perfect hard boiled eggs when camping and I have no access to a suitable utensil for piercing. If you put your eggs on to cook in cold water, bring them to the boil slowly on a medium heat and then take them off immediately and stand them for 10 minutes in the water you will get hard boiled eggs with lovely fluffy yolks.

Boiling water is boiling water, no matter how achieved.

5-7 minutes as stated above. Adjust time longer if at elevations above 1000 meters or 3000 feet (water boils at a lower temperature due to the lower air pressure). If I'm going to make 'pretty' eggs, i.e. quarter them and use them for a garnish or something, I'll put a bit of acid, usually vinegar in the boil water to reduce discoloration should the egg crack.

Note of interest: You can test an egg to see if it's hard boiled by laying it on a flat smooth surface and spinning it. If it's cooked, it will spin and spin, if liquid, will not.

No pins, no clock, no vinegar - and no cracking.

1. Cover the eggs with cold water and a lid.
2. Bring the water to boil(the whole surface of the bottom is producing bubbles).
3. Turn the stove off.
4. Wait for say 20 minutes without removing from the stove.

This method can produce slightly under- or over-cooked eggs (usually under-) depending on the stove, utensil, number of eggs, personal preferences. But is easy to tune in the first 2-3 attempts.

I have used Emeril Lagasse's method, and it works really well. I think on his television show, he called it the "rule of thirteen":

Place the eggs in a saucepan and cover with water. Season with a pinch of salt. Place the pan over medium heat and bring to a boil. Cook for 2 minutes. Remove from heat and cover with a lid. Allow the eggs to sit for 11 minutes. Drain and cool the eggs for 2 minutes in ice water. Drain and peel the eggs.