Most Zapdos are sweeper-oriented. If you end up meeting one that doesn't take more than 50% once you use Dragon Dance, odds are it is defensive and you just should pray that it either uses Roost or that you live though T-Bolt.
Yeah, but, y'know, most pokemon aren't Zapdos, but that doesn't mean I'm gonna pretend they don't exist. Also I checked, and the only way Gyarados is gonna survive a min SPAtk IV/EV Zapdos's Thunderbolt is with ridiculous EV investment in SPDef, and, well, that's not happening.
Personally I'd go with Earthquake. The *ONLY* way Gyarados will survive against Zapdos is a Dragon Danced Stone Edge (otherwise he doesn't have the Speed to get a hit in no matter what), but with a 4x weakness looming I wouldn't chance 80% accuracy. Not only that but the same argument could be applied for Earthquake against the likes of Electivire and Jolteon. You'll also want it against Dialga, since the best you'll do otherwise is a neutral hit from Ice Fang. On the other hand, both Zapdos and an opposing Gyarados are valid concerns, and Stone Edge's chance for a critical can also make a nice wall-breaker.
Well, Orb Gyarados can kill Electivire with Waterfall after a DD, and Jolteon's gonna go first and kill me no matter what, so I'm not hugely concerned about those two. Earthquake would be nice against a few guys, though, especially if I haven't had time to get a DD up. Hmm.
There are two other alternatives to consider: One is axing Ice Fang to go with both Earthquake and Stone Edge, since Grass doesn't resist Rock your only concern is a slight power loss with them and possible issues against a Stone Edge Garchomp.
Somehow that never occured to me. That's... Hmm. Hmm hmm hmm. I'll have to think about this. Hmm.
The other is, if you intend to get Platinum whenever the heck it comes out here (or know someone who imported), go with Earthquake and Outrage.
Wait what it gets Outrage? Man,
everything gets Outrage in Platinum. :/