Yes, another group approached me with this problem.
The likely culprit is that your Video class contains a javafx Rectangle object, which is more of a graphical widget than just a pure data storage object, and apparently GSON can't handle serializing it.
You may want to have a JavaFX Rectangle in order to draw it in your GUI, but inside the Video class you should probably store it as either a javafx.geometry.Rectangle2D object, or perhaps as a java.awt.Rectangle.