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