Why is it so hard for people to understand this?
If AOL is your ISP:
You use the AOL software in the GameCube games (which Nintendo has licensed) to connect to AOL. Once connected to the internet via AOL, you'll connect to the Gamespy servers and can join or start internet games.
If AOL is not your ISP:
There will be an internet setup menu in the GameCube game (similar to the one found in PSO), which you will use to configure your internet connection on your GameCube. After configuring your interent connection on your GameCube, you will be able to connect to the internet via your regular ISP (not AOL!). Once connected to the internet via your ISP, you'll connect to the Gamespy servers and can join or start internet games.
If Nintendo had not licensed this software from AOL, and AOL is your ISP:
You would not be able to play any GameCube games online, as you wouldn't be able to connect to the internet through your ISP(AOL). This is because AOL uses proprietary software to connect users to the internet.