I did not remember Gayle Sayers that well so I looked at a nfl video of his highlights. What I saw was an ability to make nice lateral cuts, good vision in the open field and the power to break arm tackles. What I also saw was a superior athlete going against defenders who were nowhere near as fast or athletic.
The comparison between the two is virtually impossible to quantify because the game is so different, but I think Kamara, like Barry Sanders and Walter Payton, are probably better than Gayle Sayers.