Where can I find GDP data of a country (or countries) for a number of years?
World DataBank from the World Bank is a great source for GDP data, but only annual data is available, not quarterly.
For quarterly data, see IMF Data from the International Monetary Fund. Use "Query Builder" to select your country, and then in "Concept," use the quick filter to input Gross Domestic Product. Choose that, and in 'Time," you can choose Quarterly, and change the date range.