Is there a way or somewhere I can see the gas used during the attack ?
Here you can see latest transactions to the child dao. This was one of many transaction the attacker used. He sent 258.05656476038172 in each transaction. Each transaction had cost of 2,848,937 GAS with gas price at 2.6e-8 Ether. The attacker has 2436828.141032285 Ethers at his account as of now. All of the transactions were identical as far as I know, So the number of transactions is:
In : 2436828.141032285 / 258.05656476038172 Out: 9443.000000000002
(There is some computer rounding error, won't explain it here but its due to float point handling). With each transaction consuming 2,848,937 gas:
In : 9443 * 2848937 Out: 26902512091
So that is your number.