Probably. However, there's several problems holding it back. The most important is Stealth Rock, since pre-Mega Gyarados is Water-Flying, making it weak to Stealth Rock. Since it needs Gyaradosite to Mega evolve, it can't hold Heavy-Duty Boots if you're playing National Dex formats. It also doesn't have any reliable recovery to help get rid of that issue, and almost all defensive Pokemon need reliable recovery to be viable. Gyarados has Rest and Sleep Talk but that's for old people who still play Gen 4 OU or even older people who love Gen 2 OU. Even if it did have recovery and no Stealth Rock weakness, it would probably still be outclassed by Mega Gyarados. You just can't ignore that power.
