GARS basically added extra fields and custom templates to a thread on a per-forum basis. It allowed you to have multiple numeric rating fields for a product (associated with a single thread). Each thread would be the "parent", and subsequent posts could either review the product by rating the various options, or just make an ordinary post.
For example, we had Martial Arts School reviews broken down with ratings such as:
*Class Size
*Equipment
*Facility
*Striking
*Grappling
All 1-10 ratings.
It was basically an overlay onto existing threads, and had options to customize the layouts of said threads. You could either use it as a Reviews, or an Article system, by customizing those meta-templates, and then assigning them to individual forums.
Here's the only screenshot of GARS that I have left; it's from a sidebar added to a forum thread displaying the ratings.
On a personal note, it boggles my mind why this hasn't been remade or even incorporated into VB itself. When we upgraded to 4.0, we essentially turned thousands of reviews into ordinary forum threads, because we lost GARS.