Ok, star ratings are awarded in price categories (based on RRP) against products reviewed at a specific time consider current trends.
5 star budget amp/speakers in most cases will not be as good as 3 star equivalents a few price brackets above. Product star ratings are also downgraded down the years for some products which doesn't make the product worse or outdated just not as attractive compared to new competition.
Not just that but subjectively many people may value 3 star amps more than their 5 star equivalents as to them they just sound better. Many on here sing praises of 5 star budget Q Acoustics and budget Marantz or Cambridge Audio gear I just personally don't like and wouldn't have for free.
And finally second hand gear loses it's value at a different rate. Which would make comparisons in star ratings virtually impossible. Especially as you're not restricted to GS range circa 2006 but also GR, GX as well as other brands from years prior.
GS20 had a 4 star review.
Against
Lack the exuberance of the best at this price level
Yet, RX6 from the next generation silver range was a 5 star product. Then again RX8 got 4 stars. They can all be picked up for similar money nowadays.
And not to mention products that were not reviewed and didn't get any stars.