In Mario Paint, the Gnat Attack minigame simply repeats whenever the player wins against the boss on Level 3. However, players who beat Level 3 may notice that a small star icon would be added to the top left corner of the screen until the minigame is exited.
The star is merely the first of a set of 15 icons that are added, one by one, every time the player beats Level 3 of a cycle. As this takes an extremely long time and does not offer any persistent reward (with the icons all being removed if the player exits the minigame), very few players would bother to beat the game 15 times in a row.
The screenshot shows what the minigame looks like with all 15 icons unlocked. Note how the "princess" icon in the middle appears to be taller than the others; it is actually made out of two separate icons. Beating the game 3 times shows the princess's head, while doing it 8 times adds the body, which appears as one icon due to touching the head.
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