If your GMod addons won't download at startup, follow this procedure to empty the server's STEAM cache. This should solve your download issue.

  1. Stop your server.
  2. Log on to your FTP server (or use the WebFTP).
  3. Delete srcds/steam_cache folder.
  4. Delete srcds/garrysmod/cache folder.
  5. Restart your server.

On reboot, your addons will be downloaded!