Vim Vertical Tab list / tab bar
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Find below a patch to use gvim with vertical tabs.
The patch only modifies the file src/gui_gtk_x11.c .
It sets the tab-bar as vertical and create a hbox to stack the tab-line left of the main window.

This was applied to vim-7.3.429

Before compiling, you can export the following to change the name of the binary file to vim-mod.
export CONF_ARGS='--with-vim-name=vim-mod '

gvim is then launched with the command: vim-mod -g

Look for the word "EDIT" in file gui_gtk_x11.c to see modifications.