First of all ignore the deposit currency for the moment.
When you initiate a trade in EUR/USD your gain or loss will always be in USD, the cross of the pair.
You are buying 10,000 units of EUR, and when you close your trade you are then selling 10,000 units of EUR, therefore your profit or loss is in USD. The base currency of the pair, (the first currency quoted) is what your buying or selling against the cross of the pair (the second pair quoted).
The profit or loss of your trade is then realized in US dollars and converted to GBP which is your deposit currency at the current rate of the GBP/USD pair.
You learn more looking for the answer to a question than you do being told the answer straight away.