Transparency from GNOME Terminal removed in version 3.8. This is a relevant bug report #695371 and there are ~400 negative comments about it on the Reddit Linux Community. It wasn’t a very popular choice, was it? ;)
What can we do?
As you can see on the screenshot the whole window is transparent including the title bar, that’s why this way is “Horrible”.
We can make all (or some) windows transparent, either automatically when GNOME Session starts, or on demand.
You can use Devil’s Pie2 or just this simple script. It is relatively easy (if you spend some time!) to use them, so I skip additional info. Terminal maintains a Frequently Asked Questions page, that you might want to check.
Alternative you can use the Transparent Windows Extension that allows setting custom opacity globaly and per window basis. You can assign different opacity to active and inactive windows. Also there is a simple mode if all you need are transparent terminals.