Is it possible to sort a list of entries by rating? for example, I would like the highest-rated entries to appear at the top of my list.

Absolutely! GME has a gm:rating control for just this purpose -- the actual rating is stored in the value attribute of the element referenced in the control's ref attribute. Say your rating control looks like:

<gm:rating ref="gd:rating"/>

you can always display the highest rated entries first by using the following gm:sort tag:

<gm:sort name="ratingSort" ref="gd:rating/@value" reverse="true"/>