Gemini 2 is unique for several reasons. Although it's mission was not one that launched men into space, it is truly one of a kind. Originally launched by NASA in 1965, it flew a suborbital trajectory intended to test the heat shield and the spacecrafts recoverability. After tests, it was given to the Air Force for testing for the Manned Orbital Laboratory program. It was launched a second time in 1966, making it the first spacecraft launched twice to space (while the x-15s made multiple trips, they flew their missions, but let us not split hairs!). As part of a military launch, the standard American flag and "UNITED STATES" was removed to be replaced by the words "US AIR FORCE" and the star and bars representing that service. Again, it's mission was to test the heat shield, but this time the shield had a hatch cut into it. Ultimately, this hatch would allow Air Force astronauts to traverse from the capsule to the orbital lab behind them without donning a space suit. Though the hatch worked, the MOL program was cut due to budget constraints and the increasing capabilities of unmanned spy satellites.
These three things - the first relaunched man rated spacecraft, the first (and only) spacecraft launched with military markings, and the first (and only) launched with a hatch in the heat shield - make Gemini 2 a unique artifact of space history.