Not for full value. Ebay or or Plastic Jungle as mentioned are the easiest way to get some cash for them.
Unless you're super hard up for cash, I'd just hold on to them since you can buy damn near everything from Amazon, it's a useful place to have credit.
Also, if they're GCs from the Amazon Visa--if you're eligible to upgrade to a platinum account you can get other rewards besides Amazon GCs--including cash back. For 5,000 points you can get a $50 check instead of getting $25 Amazon GCs every 2,500 points. As well as all kinds of other random award options--magazines, different GCs etc.