Hello,
i added my Server (62.75.188.175:27015) with the Game "Team Fortress 2"...
But this was a mistake, cause it is a "Garrys Mod" Server.

How could i change this now??

You can change the game type of a server when it is listed as down on:

http://www.gametracker.com/server_info/62.75.188.175:27015/manage/

Once the server turns online, that big red box will not be visible. If that's the case, you're welcome to claim your server and adjust the game on the above page as well.

Otherwise, we can adjust it once you turn the server back online.