Which portable GVIM are you talking about? The default installers from vim.org are not portable per se (but you can usually just copy the installation directory around and directly launch that).
If you can get admin rights, I think it's a good idea to acquire them for the one-time install process (you presumably won't upgrade Vim that often). Installing to a secured location with elevated rights reduces the attach surface: You can then be sure that nothing tampered with Vim's executable and runtime.