The tax revenue as a percentage of Gross Tax Revenue is highest from Excise Duty.

To understand this, we need to look at the options provided:

1. Income Tax: Income tax is a tax levied on the income earned by individuals and businesses. While income tax can contribute a significant amount to the overall tax revenue, it is usually not the highest percentage contributor.

2. Corporation Tax: Corporation tax is a tax on the profits earned by companies. Corporation tax can also contribute a substantial amount to the overall tax revenue, but it may not be the highest contributor.

3. Customs Duty: Customs duty is a tax on goods imported into a country. While customs duty can generate revenue, it is often a smaller percentage of the overall tax revenue compared to other types of taxes.

4. Excise Duty: Excise duty is a tax on certain goods produced or sold within a country, such as alcohol, tobacco, and petroleum products. Excise duty is often imposed at a higher rate and can generate a significant percentage of the overall tax revenue, making it the highest contributor in this case.

Therefore, the correct answer is option 4, Excise Duty, as it generates the highest tax revenue as a percentage of Gross Tax Revenue.