Here is the full list of all 22 METAGAL achievements.
Defeat Final Boss with Warp GAL
Finish the game with all characters
Defeat Final Boss with Buster GAL
Defeat Final Boss with Dash GAL
Complete the game using gal 01
Get Rank S on every level
Complete the game once
complete all levels in PAR
Die over 20 times in one level
watch intro without skip
collect more than 9 gears
beat any level in less than 3:40 minutes
defeat GAL01
defeat GAL02
defeat GAL03
defeat GAL04
defeat GAL0X
complete fortress 2
complete fortress 3
complete fortress 4
complete tutorial
Collect all powerups