World Development Indicators Online: GDP is one of the statistics under Series. You will also need to choose a country and time.

Global Insight: Enter your MIT email and click on the Data Browser tab at the top. Once in the platform, enter GDP in the search box, select the region or country you want under Geography, and click View Results.

National Statistical Offices of specific countries.

See Worldbank eLibrary. Click on Data > By Indicators.