Scrolling grids is a tricky situation, since the scroll bars are included in a different element than the grid itself is rendered in. Then you also have the issue of determining where the mouseOverPosition is as well.
I'm going to assume you can deal with the second issue for now, as that's within the com.google.gwt.event.dom.client.MouseMoveEvent.
I've heard people say that they've had luck with calling <Grid>.getView().getScroller().setScrollLeft(<arg>), or setScrollTop, but those didn't work for me personally when I needed to auto scroll a grid.
Another option is to extend the GridView class, and make "restoreScroll" a public class, which will allow you to automatically add a little bit onto the scroll.
Combine that with some pseudo code such as:
int topScrollBound = Grid.AbsoluteTtop() + Grid.getHeight();
int bottomScrollBound = Grid.AbsoluteTop() + Grid.getHeight() - 10;
If ( mouse position > topScrollBound && mousePosition < bottomScrollBound )
<code for scrolling grid down>