I'm sorry if I'm answering this incorrectly, but I assume you mean your taskbar at the top of your screen or the panel in your SuperKey.
If you want to do that, you'll want to get a theme for Gnome. In order to get a theme for Gnome, you'll have to download one here, or just look around yourself. Gnome themes change everything in your desktop except Window edges and GTK, so everything will fit in.
In order to apply Gnome Shell Themes, follow the instructions here.
If you just wanted to change the transparency of that bar alone, I don't know what to tell you, that's probably out of my pay-grade. You can do it with Unity by downloading MyUnity, which is available through the Software Center, though, and just going through the settings of it. If you try it with MyUnity, the settings might transfer over to Gnome, but there are no guarantees.
I'm sorry I could not help more, but I am considered a new user and may only use two Hyperlinks when answering questions. I'm sure a quick Google of "Gnome Shell Themes" will direct you to the right place. If you want to know a transparent theme, one is called Nord. I would have posted pics, but I'm not allowed to do that either.