Probably one of the most disgusting beasts you can encounter in Dark Souls. Gaping Dragon is giant, horrible, rotten and hideous dragon. Despite appearances, the fight with him is not too difficult, as long as you learn his behavior.
Attacks and weaknesses
Gaping Dragon has several weaknesses: his head (a small point at the top) and a tail, which can be cut off. Additionally he is vulnerable to lightning. Dragon fights in two positions - standing on all his legs or standing on his front legs with his torso raised up. In the first position Gaping can surprise you with:
1) Charge - when the demon stands on his all legs (in horizontal position), he runs towards you. In this situation you have to go out of his way, which sometimes requires sprinting and few jumps.
In other position the dragon uses following attacks:
2) Tail attack - it is a mighty swing, which can hit you when you're standing behind the beast (even in the long distance). After cutting off the tail, the range decreases.
3) Overhead - the dragon has a tendency to fly in the air and fall down with impetus. But it is easy to run away from such attack.
4) Grab - if you are close to his leg, demon can try to grab and eat you. It ends in death for weaker characters.
5) Acid throw - the dragon starts to shake and spits acid. Acid inflicts damages and damages the equipment.
6) Smashing - the beast tries to smash you with his legs.
7) Slam attack - the beast slams the ground with his upper body. This is the perfect moment to attack!
1) Cut off the tail - Gaping is one of the demons, who have a weapon in his tail - Dragon King Great Axe.
2) Do not think about the shield - blocking dragon's blows has no sense. If you play the fight well, none of them will get you and additional damage from a weapon held in two hands are very useful.
3) Stay in half-distance - in case of Gaping you have to stay not too close and not too far. The fight is to wait until the beast slams the ground with his body, so you have to be close to run fast towards him and hit him. On the other hand, if you stay too close, you can be hit by his additional blows or charge.
A tactics in this fight is very easy - you have to use two simple behaviors of the demon.
1) Charge - when the demon is charging at you, you can easily avoid his attack and find yourself at his side. From this position you can hit him once or twice.
2) Slam attack - when the demon rises up his upper body, you can be sure that he will slam the ground in a moment. After such blow, the beast is immobile for 3-4 seconds, which allows you to hit him.
Run to the arena and observe the demon. If he runs towards you, get out of his way and try to put yourself on his side. When he raises his body up, he will smash the ground in a moment.  If so, run along the beast behind his back. 
Hit the tail with a weapon held in two hands.  After few blows you'll cut it off and obtain Dragon King Great Axe. 
If the demon is far from you, he will start charging again. Get out of his way and jump aside.  After next ground slams, the dragon is defenseless. Hit him 3-4 times in any part of his body. But after that run away - he will start charging at you again. 
From time to time Gaping flies up - try to stay away from him, when he does so.
After killing the demon, pick up Hard Leather armor, lying on the arena.