The following are known Action Replay Codes for Gradius Galaxies on Nintendo Game Boy Advance (GBA).
These codes were also known as GameShark Codes and in most cases will work on both devices.

These codes are for the U.S. version of the game.

Master Code – Must Be On
b823843b
a1f907ab
5ba14bfa
015a315f

Infinite Lives
facf9352
ec275fea

All Levels
94aecc25
a2a81f77

Max Score
b7ce538c
348343ec

Just Have Speed Up
ba0e2376
8f7cb9f0

Just Have Missile
1ddac054
4ec63d4a

Just Have Double
f4837669
bef3c2de

Just Have Question
abf8fa01
c342e257

Have All Power Ups
cddf06ff
3a67cec9

Just Have Laser
bb7e145a
e31edca6

Just Have Option
5eacf517
c8e4a39c