GATT was a series of multi-lateral treaties -- and an organization of the same name -- designed to regulate trade between the member nations.
The main GATT treaty is posted on the Internet by the World Trade Organization and Tufts' Multilaterals Project.
The GATT Digital Library posts a substantial collection of GATT documents and publications (including treaties and other legal instruments).
GATT Panel Decisions are available on Westlaw (WTO-DEC). GATT principal decisions, resolutions, recommendations and reports are available on Lexis (ITRADE;BISD).
The original GATT organization and agreements (which regulated only trade in goods) were incorporated into the much broader World Trade Organization in 1995 by the Uruguay Round agreements, which created the WTO to regulate trade in goods, services and intellectual property.
For more information and resources, see NYU's WTO and GATT Research page.