Includes Gaiden (the best Gradius game) as well as the first four games.
Gradius III is like the hardest game ever
Some things to take note:
Gaiden is only one player (the PSX one was originally 2 player)
The defaults are "dumbed" down with small hit box and on 'easy' difficulty where original defaults didn't have the "hitbox" feature - full sprite was hitbox
Also 'wait' is turned off - for arcade accuracy, set 'wait' to '2'
The PSP d-pad is known to have some issues with diagonal movements… I personally have not had this issue though many have and there are modifications that can be done by searching google for PSP d-pad mod.
Also look for:
Salamander Portable(also has Xexex!)
Twin Bee Portable