The gp_stadium.zip file you have requested: Garry's Mod / Maps / Half-Life 2: Garry's Mod: Stadium Map - 895.54kb is not available. Perhaps try browsing all files for the game Half-Life 2 and upload any files you want to share or are missing. This is the only information we could locate:
This is a racing map for Garry's Mod. It's a dirt track with tight turns and nice jumps. There is also a snow version of the map in the zip file.