In X-Plane, a couple of extra steps are required to align the RealSimGear display.
Windows
Don't worry though as X-Plane will remember the location for the next time you start your sim. You may find this process much easier if you move your taskbar to either the left or right side of your screen. The taskbar creates a snap point which makes hiding the white bar difficult on some systems.
Follow these steps, or check out the videos below.
Click the center of the GPS screen to pop out the window
Click the small rectangle at upper right of the popped out window to detach it
Drag the window approximately to the RealSimGear device screen
Drag the left and right edges to align with the RealSimGear display
Make the window taller
Drag the window down to align to the bottom edge of the RealSimGear display
Drag the top edge to make it shorter, then without letting go of the mouse button drag up so the top edge is just a few pixels above the edge of the RealSimGear display
Click in the main window
Mac
Click the center of the GPS screen to pop out the window
Click the small rectangle at upper right of the popped out window to detach it
Drag the window approximately to the RealSimGear device screen
HOVER you mouse over the green maximize icon in upper left corner of the relocated pop-out window and choose "Tile Window to Left of Screen"
Click anywhere on the instrument on the RealSimGear screen to make it fill the screen with title bar hidden.