Where is the trap part?
This is an olschool gambit, which is refuted by 4.Nxe5 Nxe5 5.d4 Ng6 6.e5 Ng8 7.Bc4 c6!
And curiously enough, playing the same thing as Black a whole tempo down (1.e4 e5 2.Nf3 Nc6 3.Nc3 Nf6 4.g3 Nxe4!?) is entirely playable!
The Four Knights Game: Halloween Gambit Trap
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The Halloween Gambit (also known as the Müller–Schulze Gambit or Leipzig Gambit) is an aggressive chess opening gambit in which White sacrifices a knight early on for a single pawn. The opening is an offshoot of the normally staid Four Knights Game and is defined by the moves:
The theoretician Oskar Cordel reported in 1888 that Leipzig club players used the opening to dangerous effect, but he did not believe it was sound. Their name for it, Gambit Müller und Schulze, was not after any players by those names, but rather a jocular German equivalent of "Smith and Jones" or "Tom, Dick, and Harry". The modern name "Halloween Gambit" was given by the German player Steffen Jakob, who explained that "Many players are shocked, the way they would be frightened by a Halloween mask, when they are mentally prepared for a boring Four Knight's, and then they are faced with Nxe5.
White's objective is to seize the center with pawns and drive back Black's knights. After 4... Nxe5, White usually plays 5. d4 (5.f4 does nothing for development), after which Black can retreat the attacked knight to either g6 or c6.