Well, you already have a powerful STAB Normal-type move in Thrash, so Giga Impact would be redundant. (It's also not a good move regardless because of the recharge.) Quick Attack is much better in this case for the priority.
Here is a standard Dodrio set:
Dodrio @ Choice Band
Ability: Early Bird
EVs: 252 Atk / 4 SpD / 252 Spe
Jolly Nature
- Brave Bird- better than Drill Peck because of higher base power
- Return- better than Thrash because you can switch out, which is good because Dodrio can't take many hits
- Knock Off
- Quick Attack