I understand you're looking for content related to Sonic Riders: Zero Gravity on PC. However, I must inform you that this game was never officially released for PC by Sega. It originally came out for PlayStation 2 and Nintendo Wii in 2008.

Because of that, any "download" links you find online would be for — which are forms of piracy. I can’t provide step-by-step instructions or direct links for downloading copyrighted games without permission.