This was a Gradius compilation for PlayStation Portable. This compilation contains the classic versions of Gradius I-IV with a few bonus features thrown in as well as the first international release of Gradius Gaiden.
The first true Gradius game to introduce the concept of the ‘weapon bar’. During the game, many enemy craft leave behind icons or ‘pick ups’ when destroyed. Collecting one of these will shift the selection cursor along the weapon bar at the bottom of the screen. The player can then select the weapon highlighted if they want it.
The cursor then resets. In general, the more useful ‘power ups’ are towards the right hand side of the bar, so the player may decide to stock up on pickups until the better item is available.
This innovation allowed for deeper tactics on the part of the player and for greater freedom of weapon choice rather than relying on the pre-determined power ups common in other games in the genre.
Gradius II (1988)
Gradius II is the sequel to Gradius in terms of chronology. The game did not see a North American release until 2006 as part of the PlayStation Portable title Gradius Collection.
Gradius III (1989)
This title introduced the Weapon Edit method of selecting weapons, which allowed players to create their own weapon array by choosing power-ups from a limited pool of available weapon types (some weapons in the preset weapon types are not selectable in Weapon Edit mode, although it includes weapons not in any presets).
The difficulty and major boss tactics were toned down to make it easier. The original arcade version is available for PlayStation 2 bundled with Gradius IV (Gradius III and IV), although the port has some slight differences from the original.
Gradius IV (1999)
Released in Japanese arcades as Gradius IV Fukkatsu. IV lacked the Weapon Edit function of its predecessor, but it had a bigger array of weaponry than the original Gradius games. Weapons exclusive to this game included the Vertical Mine missile (which detonates in a vertical line shortly after deployment) and the Armor Piercing laser (a shorter-ranged, more powerful laser).
It was released on the PS2 in a compilation pack together with the arcade version of Gradius III (Gradius III and IV).
Gradius Gaiden (1997)
The first Gradius produced exclusively for a home console. This is also the only Gradius game where players can select which ship they wish to use. Gradius Gaiden includes the Lord British Space Destroyer from Salamander and two (relative) newcomers: the Jade Knight and the Falchion β (a variation of the ship from the Famicom Disk System game Falsion).
It was originally released for the PlayStation console and ported in 2006 as part of Gradius Collection for the PlayStation Portable. In 2019, it was included in the Japanese version of the PlayStation Classic mini console.
Here are a couple of screenshots
Here is some basic gameplay