Double taxation & gross receipts tax

Why am I paying taxes twice on the same income?

You're not!

Income taxes and gross receipts taxes are different taxes. Income tax is paid on the profit a business earns (sales less business expenses); gross receipts tax is paid on the sale amount.

I earn commissions on sales on which the company I represent pays GRT. Isn't GRT collected twice?

Maybe

Commissions involve 2 separate transactions which both of which may be subject to gross receipts tax. The first is the sale of a product or service on which the seller must determine whether the sale is taxable. The second there is the commission paid to the sales representative for facilitating the sale. Note that if the sale is not taxable, then the commission is also deductible.