Is there a way to let gVim only run a single instance, so that when a new file is opened with it it\'s automatically opened in a new tab in the currently running instance?>
It depends on your operating system and shell. Using linux you can always set up an alias like:
alias gvim='gvim --remote-tab-silent'
in your .bashrc (if you use bash as your login shell).
.bashrc
On windows see the Vim wiki for solution: http://vim.wikia.com/wiki/Launch_files_in_new_tabs_under_Windows .