These tips are all variations on the same theme :
1) On Parsec 1, allow all of the enemies to appear and fly into formation without shooting any of them. After the boss alien appears (the one that carries the tractor beam attachment), you'll see a pulsar similar to the ones the aliens appear from fly from just above the boss alien to the middle-right of the screen. Once it's gone by, shoot only the second alien from the left in the bottom row of the formation. A 'Special' flag will appear just below where it was; pick it up and you'll get a bonus ship.
2) In Parsec 1, go up to the very front and wait for all the bugs to come out. Then kill the 2nd bug from the left in the bottom row to make the special flag appear. Now, when the boss is coming down, move your ship and kill yourself by touching it. Don't touch the blaster head, just the boss. Your ship is turned into the blue one, and go get the special flag.
Another Approach To The Same Situation : At the start of the Parsec 1, push straight up until you stop. DON'T shoot! Just sit there and let all the bugs form. Right after the last row of bugs get into formation, a star will shoot from the top left side of the screen to the bottom right. If you time this right you can get up to five shots on it. After about ten hits on the star (it will come out at random times during the game) it will transform your ship into a new ship that can shoot three shots on the screen (three shots vertically in a row) instead of just two.
Easter Egg :
1) Enter service mode.
2) Keep Start and Button 1 pressed, move joystick Left until sound reaches 19. '(c) 1984 NAMCO' will appear on the screen.
In the high score entry of Gaplus, you have to enter a name, age, and blood type. There are two codes which can be entered here, but cannot be entered in the international versions of the game.
This makes a hidden message appear onscreen that will not disappear until the game is power-cycled. Here is the translation from Japanese:
"You have beaten Gaplus. In commemoration, we send you this message. Send a letter to Kojima at Namco, and he'll give something to the first 10 respondents. Sorry, but turn off the power now."