I would try to generalize the cost of the gowns with a variable, say --- "g"!
A 25% increase is 1.25 (1 + 25/100), so the final cost of one of the gowns would be 1.25g.
A 25% decrease is 0.75 (1 - 25/100), so the final cost of the other gown would be 0.75g.
Adding these two together, it gets 2g, which also happens to be the original cost of both:
Thus, as the result of these transactions, the percentage gain/loss = 0%