Say that GBP USD trades from 1.1510 to 1.1511 then you have made one pip in profit. If you bought 10,000 GBP vs. the USD at the above-mentioned prices then you would have made 1 USD, because 10,000 x 1.1511 = 11511, and 10,000 x 1.1510 = 11510, and $11511 - $11510 = 1 USD.