if this is still a problem, i have observed something that may be related: steam_ugc_get_item_install_info() seems to only work if you run it from steam now. it doesn't work if you run the game from within gm studio; you need to compile it, upload it to steam, and only then will it work.
edit: nevermind, i just tested it with steamworks v1.33b, and it doesn't seem to work even when the build is uploaded to steam. there seems to be a bug with this. i hope this is fixed quickly! perhaps i should revert to the older version of gm until this bug is fixed, as it was working fine before.