Gordon is a good player, but he is well below what I would consider elite. He has only had one 1,000 yard season, and one season over 4 ypc.
Gordon is a back I would seriously consider not giving a second deal too. I wouldnt be staunchly against it because he hasnt had a huge workload. He is also starting to develop the utility that had Matt Forte playing at a high level through his second contract. But at the price it may cost, I dont know that he will bring that value to a team.
If I was SD, I would probably look to let him walk and replace him with a back picked in the first 3 rounds