BlackBerry is not support Animated GIF, especially Animated GIF on (Bitmap) Field.
You can extend BitmapField to create your own AnimatedGIFField[1].

public class AnimatedGIFField extends BitmapField
{
    private GIFEncodedImage _image;     //The image to draw.
    private int _currentFrame;          //The current frame in
                                        the animation sequence.
    private int _width;                 //The width of the image
                                        (background frame).
    private int _height;                //The height of the image
                                        (background frame).
    private AnimatorThread _animatorThread;

    public AnimatedGIFField(GIFEncodedImage image)
    {
        this(image, 0);
    }

    public AnimatedGIFField(GIFEncodedImage image, long style)
    {
        //Call super to setup the field with the specified style.
        //The image is passed in as well for the field to
        //configure its required size.
        super(image.getBitmap(), style);

        //Store the image and it's dimensions.
        _image = image;
        _width = image.getWidth();
        _height = image.getHeight();

        //Start the animation thread.
        _animatorThread = new AnimatorThread(this);
        _animatorThread.start();
    }

    protected void paint(Graphics graphics)
    {
        //Call super.paint. This will draw the first background
        //frame and handle any required focus drawing.
        super.paint(graphics);

        //Don't redraw the background if this is the first frame.
        if (_currentFrame != 0)
        {
            //Draw the animation frame.
            graphics.drawImage(_image.getFrameLeft(_currentFrame), _image.getFrameTop(_currentFrame),
                _image.getFrameWidth(_currentFrame), _image.getFrameHeight(_currentFrame), _image, _currentFrame, 0, 0);
        }
    }

    //Stop the animation thread when the screen the field is on is
    //popped off of the display stack.
    protected void onUndisplay()
    {
        _animatorThread.stop();
        super.onUndisplay();
    }


    //A thread to handle the animation.
    private class AnimatorThread extends Thread
    {
        private AnimatedGIFField _theField;
        private boolean _keepGoing = true;
        private int _totalFrames;     //The total number of
                                        frames in the image.
        private int _loopCount;       //The number of times the
                                      animation has looped (completed).
        private int _totalLoops;      //The number of times the animation should loop (set in the image).

        public AnimatorThread(AnimatedGIFField theField)
        {
            _theField = theField;
            _totalFrames = _image.getFrameCount();
            _totalLoops = _image.getIterations();

        }

        public synchronized void stop()
        {
            _keepGoing = false;
        }

        public void run()
        {
            while(_keepGoing)
            {
                //Invalidate the field so that it is redrawn.
                UiApplication.getUiApplication().invokeAndWait(new Runnable()
                {
                    public void run()
                    {
                        _theField.invalidate();
                    }
                });

                try
                {
                    //Sleep for the current frame delay before
                    //the next frame is drawn.
                    sleep(_image.getFrameDelay(_currentFrame) * 10);
                }
                catch (InterruptedException iex)
                {} //Couldn't sleep.

                //Increment the frame.
                ++_currentFrame;

                if (_currentFrame == _totalFrames)
                {
                    //Reset back to frame 0 if we have reached the end.
                    _currentFrame = 0;

                    ++_loopCount;

                    //Check if the animation should continue.
                    if (_loopCount == _totalLoops)
                    {
                        _keepGoing = false;
                    }
                }
            }
        }
    }
}

[1] http://www.blackberry.com/knowledgecenterpublic/livelink.exe/fetch/2000/348583/800332/800505/800345/How_To_-_Display_an_animated_GIF.html?nodeid=1405903&vernum=0