<C-i> and <Tab> are strictly equivalent. You are actively overriding <Tab> and therefore <C-i> in ~/.vim/plugin/settings/Settings.vim. If you want <C-i> to work normally don’t override it (or <Tab>). Simple. Unless you have a very specific reason to do so, you don’t need to put anything in ~/.gvimrc. The normal place for your settings/mappings is ~/.vimrc, there’s no valid reason to put them anywhere else (~/.vim/plugin/settings/Settings.vim or wherever). Use nnoremap <C-+> <C-o> if you want <C-+> to do what <C-o> does, whatever benefit you think you will get from doing that.
Tab和Ctrl+i是对等的, 可能是Tab键被重映射了, 到.vimrc检查一下, 去掉这个重映射就可以了.