(G)Vim transparency the easy way

Xcompmgr comes with a command that enables you to turn any window transparent, you can execute that command when Gvim is turning on by placing it in the ~/.gvimrc file or by executing the following in Gvim:

:!transset -a --dec 0.15

In some distros transset is named transset-df and it's included with xcompmgr (X composition manager)
https://www.freedesktop.org/wiki/Software/xapps/

enjoy!