Print the insets Toolkit.getDefaultToolkit().getScreenInsets(gc) on a machine with Gnome 3. For instance, run (or slightly modify and run) the test java/awt/Multiscreen/MultiScreenInsetsTest/MultiScreenInsetsTest.java.
The test gets insets and screen bounds of all devices in turn and displays and maximizes a Frame on each of them.

I have OEL 7 (Gnome 3) with 2 monitors.

The first and default screen has top and bottom toolbars however the insets reported are always all 0s for all screens.
Note that maximized Frame does know about insets and has the correct size in both cases.

It is NOT true for single-screen configuration, however.
It isn't true for Xfce4 w/o Xinerama.