Gortex windstopper is a windproof, not a waterproof product. The classic gortex waterproof/breathable fabric is something else.
Goretex waterproofness lasts for many years of normal use. That is not the problem.
The surface treatment, which causes water to bead and run off does wear out. Once the surface fails to bead, the water soaks into the nylon. The layer of goretex is sound, but becomes waterlogged and fails to breath. Beading can be restored in some cases by a warm iron, or by sray-on stuff.
The surface also becomes clogged by dirt, esp the oily water from road muck.
My advice to goretex users is to use your waterproof only when it is raining. When you need a windproof, use a cheaper, more effective windproof (one of the modern microfibre fabrics). Keep your gortex jacket clean inside a plastic bag, inside your pannier.