Two years ago, in GNOME 3.8, background transparency was removed from gnome-terminal. Over the years several users expressed their fondness for this feature, leading to much drama, tricky workarounds, and rosy promises.

Anyway, this is now back in Fedora 20 and the GNOME 3.12 COPR. Rawhide will follow soon.

I must point out that this is a downstream patch carried by Fedora, which has been rejected by the upstream gnome-terminal maintainer. If you want, you can ask your distributor to include it. Versions of the patch applicable to different GNOME branches can be found in this Git tree. You will also need a vte that has the fixes for bug 730023 and bug 729884 depending on the branch that you are using.

Be aware that this has exposed a bug in Adwaita where it is not drawing the background of the menubar when transparency is turned on. We are looking into it and hoping to fix it soon.