The major source of gross tax revenue (GTR) for the Government of India is service tax. Service tax is a tax levied by the government on services provided by service providers. It is a form of indirect tax.

Income tax, option 1, is a direct tax levied on the income of individuals or entities. While income tax is an important source of revenue for the government, it is not the major source of GTR in this case.

Corporation tax, option 2, is a direct tax levied on the net income of companies. Similar to income tax, it is an important source of revenue for the government, but not the major source of GTR.

Customs duty, option 3, is a tax imposed on goods imported into or exported from a country. It is also an important source of revenue for the government, but not the major source of GTR.

Therefore, based on the options provided, the correct answer is option 4, service tax, as it is the major source of GTR for the Government of India.