NJ, both the import tax and the export tax would affect business so one isn't a solution to the other, they both make things worse.
Of course GDP is relevant. If your plan is 'childish tit-for-tat' the only thing that matters is the relevance of the amount your are talking about, not the amount itself. It is the affect of that trade against your overall trading figure. I don't have the figures, but say the EU had five times our GDP, then a million pounds worth of trade lost to each of us would be five times more important to us.