Which is the particular agency in WTO which was absent in GATT?

The previous poster is on the right track. The GATT was not an official organization, but rather an informal agreement among countries to address trade barriers that sprung up during WWI and WWII. There were many attempts to create an actual organization -- based on the structure of other organizations, like the IMF or the World Bank -- that would work on extending agreements on trade to other countries and extending the agreements to cover more goods. This finally happened when the WTO was created, and WTO has broadened the treaties of GATT in several ways.

This is perhaps a trick question? Of course you know the GATT still exists for the trade in goods? There are too many contrasts for this to be a real question. The whole of GATT was revised.

If you are serious simply review the Wikipedia question and aim the question at illuminating a particular point please.