Gyarados is definitely worth training. With that high of attack you can hardly go wrong. Like unidenified said, he has a high disadvantage to electricity, but with his water AND flying types he doesn't have many weaknesses.
Personally I wouldn't teach my Gyarados any of those moves except for maybe Earthquake and Waterfall. Since he already has really high attack and Dragon Dance wouldn't do much use, except to maybe raise his speed a bit. Here's a better move set:
Waterfall (good STAB)
Ice Fang (yeah this is my choice instead of Stone Edge)
Earthquake (great coverage)
Hope this helps =)