GameCube was the Tony Hawk's Pro Skater 3 platform that served as a North American launch title.
Nintendo released the GameCube in North America on November 18, 2001, and Pro Skater 3 was among the games available for the new console. That timing made the GameCube version a launch title, giving players an established multiplatform sports game alongside Nintendo's first-party releases and other launch-period software.
The wording can cause confusion because Pro Skater 3 also appeared on PlayStation 2, the original PlayStation, Game Boy Color, Game Boy Advance, Xbox, Windows, Mac OS, and Nintendo 64. Those platforms were not the North American GameCube launch platform described in the question. The PlayStation 2 version is particularly easy to mix up because it was the first game in the series to support online play on that console.
Pro Skater 3 was released in 2001 and became one of the most highly regarded entries in the series. Its GameCube edition retained the core skating, goals, levels, and professional roster, while the Xbox version later stood out for technical improvements and an additional Oil Rig level.