Its not and never has been a part of standard moodle - so someone at some time installed it, or it came as part of a pre-built package from Gnomio's own installer.
If the permissions are set appropriately you should be able to install from the front end (Site Admin > Plugins) assuming you cant ftp onto the back-end of your site to install it.
Otherwise, you probably need to speak to your hosts, but quite possibly, if your site is now on 3.5 and that theme is only available upto 3.2, that may be why they wont allow it to be installed.