Click the needed HTML or Google Gadget file in the Workspace panel.

Go to File → Open With... from the top menu.

Select the WYSIWYG editor, and click Open to show the selected file in the WYSIWYG editor;

Or, double-click the WYSIWYG editor.

If the target file has been opened in the Content panel before, the confirmation message will be shown. Click Yes to accept reopening the file.

Your selected file will be opened in the WYSIWYG editor as below.

If you want to change the default editor, simply select your desired editor and tick the Use as default editor checkbox in the Open File With form.

The new settings are stored in the User Settings File at the server. The selected editor will be the default editor in next sessions. The default editor is marked with the "Default" text in the Open File With form.