Hayden averaged 43 in ODi cricket, and the only reason Ganguly scored more hundreds is because he played over double the amount of games as Hayden. Hayden's got a better average, and a better strike rate and in his prime in ODi cricket was unstoppable. World Cup 2007 he was glorious, hell for the whole of 2007 he was supreme.
I just don't rate Gambhir that highly from what I've seen of him. He's a very talented young guy, but I do not rate him anywhere near as highly as Tendulkar. I rate Sehwag far higher than Gambhir. He had a good year last year and has started well this year, but I don't think he'll ever be regarded as anywhere near Sachin in terms of his record or his ability. Against the top sides in the world his average drops considerably. He only averages 32 with a strike rate of 81 and although he's still performed well in his last 2 seasons, his record last season wasn't superb, averaging just over 40 striking at 84, he's certainly no Tendulkar. He'll probably end his career known as a very good player, but not in the same bracket as Tendulkar, Hayden, Gayle, Ganguly or Gilchrist.