You have implemented your own graphical editor as a new eclipse plugin. You are looking for a way to add zoom functions into the graphical editor. It is very easy!
1. Add the action into the toolbar:
...in Class: ActionBarContributor...
    ...in Method: contributeToToolBar...
        toolBarManager.add(getAction(GEFActionConstants.ZOOM_IN));
        toolBarManager.add(getAction(GEFActionConstants.ZOOM_OUT));
        toolBarManager.add(new ZoomComboContributionItem(getPage()));

2. Connect the Zoom Manager with your edit part:
...in Class: YourEditor...
    ...in Method: configureGraphicalViewer...
        ScalableFreeformRootEditPart rootEditPart= new ScalableFreeformRootEditPart();
        getGraphicalViewer().setRootEditPart(rootEditPart);

        ZoomManager manager = rootEditPart.getZoomManager();
        IAction action = new ZoomInAction(manager);
        getActionRegistry().registerAction(action);
        action = new ZoomOutAction(manager);
        getActionRegistry().registerAction(action);

        //define the zoom possibilities
        double[] zoomLevels = new double[] {0.25,0.5,0.75,1.0,1.5,2.0,2.5,3.0,4.0};
        manager.setZoomLevels(zoomLevels);
   ...in Method: getAdapter...
        if (type == ZoomManager.class)
              return ((ScalableFreeformRootEditPart) getGraphicalViewer().
                        getRootEditPart()).getZoomManager();

That's all! Now you have two buttons to zoom in and zoom out. And you can see the zoom level in percent.  With the example here you  can change  zoom level from 25% to 400%.

There is an example of creating linked resources in code.

IProject project = workspace.getProject("Project");//assume this exists
   IFolder link = project.getFolder("Link");
   IPath location = new Path("C:\TEMP\folder");
   if (workspace.validateLinkLocation(location).isOK()) {
      link.createLink(location, IResource.NONE, null);
   } else {
      //invalid location, throw an exception or warn user
   }

This example will create a linked folder called "Link" that is located at "c:\temp\folder".

link.getFullPath() => "/Project/Link"
link.getLocation() => "c:\temp\folder"
link.getRawLocation() => "temp/folder"
link.isLinked() => "true"
IFile child = link.getFile("abc.txt");
child.create(...);
child.getFullPath() => "/Project/Link/abc.txt"
child.getLocation() => "c:\temp\folder\abc.txt"
child.getRawLocation() => "c:\temp\folder\abc.txt"
child.isLinked() => "false"

Recently I had a problem in converting java.io.File to IFile for external files. The external files sind the files which are not located in the eclipse workspace. So they are not eclipse "resources" and can not be converted into IFile instances. With the following code you can solve this problem:

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                 IEditorInput input= createEditorInput(selectedFile);
                 String editorId= getEditorId(selectedFile);
                 IWorkbenchPage page= fWindow.getActivePage();
                 try {
                     page.openEditor(input, editorId);
                 } catch (PartInitException e) {
                     //EditorsPlugin.log(e.getStatus());
                 }

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This idea comes from org.eclipse.ui.internal.editors.text.OpenExternalFileAction. In this class you can also find the implementation for the methods "createEditorInput" and "getEditorId".