Here is the full list of all 21 Gun Wings achievements.
Finish the second stage on any difficulty
Finish the first stage on any difficulty
Absorb enemy bullets to gain energy
Finish the third stage on any difficulty
Finish the fourth stage on any difficulty
Finish the fifth stage on any difficulty
Finish the sixth stage on any difficulty
Use the bomb at least 10 times
Score 10,000,000 points in one life
Score 30,000,000 points in one life
Score 50,000,000 points in one life
Die while using charge-attack
It doesn't help
Get 1,000 gems in a random stage
Get 3,000 gems in a random stage
Get 5,000 gems in a random stage
Unlock nightmare difficulty
Unlock practice mode
Prove that you can defeat the aliens
Die 44 times in a single game
Collect some points while charge-attack mode is active