One of the touted benefits of Brexit was that it is a big wide world out there and by leaving the EU we would no longer be held back and would able to exploit the opportunities on offer.
Now it turns out that a trade deal with our largest trading partner (in terms of exports) is not all it's cracked up to be and that economically we'd probably be better off if everyone looked down the back of their sofa and spent the change they found.
GDP like all figures is only useful as a tool if it is used properly. The comparison, in this case, as to how certain outcomes will influence UK GDP is perfectly valid - hence our own, pro-Brexit, government using them.
Other factors will influence GDP and it is possible that even with a fall in GDP the average UK citizen will not notice any difference to their lives - it may even be the case that despite a contraction in GDP tariff reductions or other stimulus means that the average person is slightly better off - but figures cannot be dismissed because on their own they do not tell the whole story or because they do not give the desired result.
But ultimately this line of argument is no more sophisticated than economic predictions are complicated therefore just ignore them.