@statox Great! I thought --remote-silent might do what you want, but it doesn't open a new buffer, it replaces the active one.

@Carpetsmoker "Sort of". If you have an existing gVim session, the files opened by this method will be added as new tabs (and if you have multiple sessions, they will be added to the most recently created). But it doesn't prevent you from opening multiple sessions using other methods (eg, running gvim.exe again, right-clicking and editing, etc).