"Worthwhile" depends on several factors, such as what other pokemon you'd like to spend that stardust on and how easily you can farm magikarp for another gyarados. What I can answer is whether this gyarados is better than a lower IV one with better moves:
Good moves are far, far more important than IVs. The difference between 100% and 0% IVs for a level 25 gyarados is only about 250CP, and that's the most extreme case. The difference between a perfect gyarados and a good (~80%) gyarados is less than 50CP.
In comparison, having hydro pump instead would offer a massive increase in damage output. Even dragon pulse would still more than make up for losing a bit of CP- I'd still consider my weaker gyarados with lower IVs better than yours.
I'm not factoring in bite vs dragon breath as both have practically the same speed and damage, with no STAB. It comes down to what you're fighting most.