Your GPanelWithText class  should not extend from JScrollPane, rather it should
extend from JTextArea and should be inserted inside of a JScrollPane, like so:

GPanelWithText gPanel = new GPanelWithText();
frame.getContentPane().add(new JScrollPane(gPanel));

You can still configure the scroll pane if you want, i.e.:

GPanelWithText gPanel = new GPanelWithText();
JScrollPane scrollPane = new JScrollPane(gPanel);
scrollPane.setVerticalScrollBarPolicy(JScrollPane.VERTICAL_SCROLLBAR_ALWAYS);
frame.getContentPane().add(scrollPane);