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%.