You always use the base currency in the pair to determine position size. In the case of EUR/USD, as the example showed, the position size was in EUR. You multiplied the position size by the exchange rate to get the USD value.
For a cross, it's the same deal, but you need to remember what to multiply by. In your GBP/JPY questoin, the base currency is GBP so that's what your position size is base. To get the USD value of the position you need to mutiply through by the GBP/USD exchange rate.
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